As with everything in the game, military units have a level, which is equivalent to every other level in the game (except for spells and magic items which are ranked instead). This will be used for various purposes such as determining the actual HD/level of the unit commander, or determining the extent of an event affecting the military (a local event for example will have a level of 1d6, whilst a major event [such as a war] will have a level of 3d6). Note that the military hierarchy is a social hierarchy that usually has precedence immediately after the aristocratic hierarchy. Officers will have a default social level in this hierarchy based on the level of their command, although regimental officers have a +1 bonus to their social level and general officers have a +2 bonus to their social level. That said, most senior officers will actually have a higher social level due to their membership of the aristocratic caste.
Please note that this is a rather generic table of organisation which will often vary considerably in practice, especially with regard to the number of soldiers in a company. For example some elite units might be very small, whilst some of the cheaper infantry units might be quite large (twice or even four times the number), but each of these, despite the difference in size, will still only count as a single company.
The following table is the actual heart of this post (also see the section on Civil Improvements below). That's everything you absolutely need to know. Everything else may be considered excessive verbiage.
|11||Division||Lt/Mj General||[1/3rd Army]|
|12||An Army||General||Typically 6 Regiments|
|12+||The Army||[Field] Marshal||Typically 1-3 Armies|
The three most important unit sizes are the company, the regiment, and the army. Lets start in the middle (because it will actually make more sense doing it this way). This discussion will focus on the armed forces of kingdoms; those of empires are an advanced topic (especially since an empire is often an empire because it conquered neighbouring kingdoms, which means it tends to have a more specialised military).
The armed forces of a kingdom may be divided into two types of forces. The first are the garrison forces that are considered organic parts of the various defensive structures in the kingdom. The second are the mobile forces that may be deployed on campaign, and which effectively form the standing army of the kingdom.
The standing army of a kingdom is composed of a number of regiments (up to two per county). These may be standing royal regiments paid by for, and under the direct command, of the king, or feudal troops under the command of each region's count (as the personal feudal representative of the king for that region). Most kings prefer a standing army that is answerable only to them, whilst most nobles prefer that the troops owe loyalty to the noble first (who is, of course, the king's most humble and loyal servant).
Soldiers will primarily identify themselves by the regiment they belong to, even if they are not members of a formal regiment (for example those feudal troops raised by the Count of Solisberry will consider themselves Solissberrians, even if there is no actual formal Solisberrian Regiment. Since regiments are generally raised and supported directly by each county, even if they are in direct service of the crown, there will have strong regional identities (and loyalties) anyway.
A regiment will consist of a number of different troop types, although they will be primarily composed of either infantry or cavalry. You will notice that the above table lists two different levels for a regiment and its commander. A level 9 regiment is generally the more expensively equipped cavalry or guard regiments, whilst the level 8 regiment is generally an infantry regiment. The commander of both is referred to as a colonel (in later militaries the junior colonel may be given the official title of lieutenant colonel, but will still customarily be referred to as colonel).
A regiment will traditionally consist of three battalions: the vanguard, mainguard, and rearguard. Formally this refers to the order in which these forces march and how they deploy on the battlefield (the vanguard to the right, ant the others to the left of it in order). Each of these is lead by a major (the battalion commanders). These are the senior regimental officers. Other regimental officers, such as the martial magistrate or regimental quartermaster, are also considered to be majors (if only for the authority this gives them other the captains of the companies that make up the body of the regiment.
Each of the battalions will generally consist of 2-4 companies, each commanded by a captain. A company is composed of similarly equipped troops that fight together on the battlefield as a single tactical unit. As for the regiment, the level of each company (and its captain) is determined by its type. A guards or cavalry company will be commanded by a level 6 senior captain, whilst a standard infantry company or an artillery company will be commanded by a level 5 junior captain.
Finally it is possible that additional companies may be attached to the regiment as support elements, but not considered to be part of any of the battalions. These will either deploy on the battlefield with the regiment. They are generally commanded by junior captains. An example is the catapult-equipped artillery company that is normally part of a Roman legion. Note that the ballista squads attached to each century are technically under the regimental command of both the captain of this company (even if they are directly associated with each century and normally accompany them.
A company is a unit of troops that are generally armed and equipped identically and which generally functions on the battlefield as a single tactical unit. As with the regiment, the exact level of the company and its commander is generally determined by the type of company it is. Infantry and artillery companies are generally commanded by a junior captain (of level 5), whilst cavalry and guard companies are generally commanded by a senior captain (of level 6). There are exceptions of course. For example the double-sized centuries of the first cohort in a Roman legion are commanded by senior centurians (the senior-most centurian being the primus pilus or "First Spear.") Again regardless of the actual level, the officer is just referred to customarily as a 'captain."
Similar to the regiment, and for much the same reasons, the company is divided into three troops (or platoons or sections), each under the command of a lieutenant. These are again the vanguard, mainguard, and rearguard, which describes the position they march in in column and the position they occupy on the battlefield (with the vanguard taking the right wing and the remaining troops deploying to the left of that in order). The senior-most lieutenant commands the vanguard, and seniority descends from there.
Other officers (such as those leading support elements like an attached artillery detachment) may also carry the rank of junior lieutenants. Finally the trainee officers of the company will have the rank of ensign (which is effectively a level 3 rank that that is actually inferior to the rank of sergeant). Note that if it is not a leadership position the commander of a support detachment will carry the equivalent rank of a sergeant (level 3). The unit armourer for example is considered the equivalent of a sergeant (as well as being a master craftsman, also a level 3 position in the guilds hierarchy).
Discipline in a company is maintained by the sergeants, who are typically large burly men who are good with their fists. Generally there are two sergeants in each troop, although they generally form their own squad (mess), rather than being embedded with the troops. The senior-most sergeant would hold the rank of company sergeant major and would technically represent the company's soldiers on the command staff. Note that sergeants will generally not lead troops into battle (in fact their normal position in battle or march is in a supernumerary rank behind the troops, and their responsibility in a battle is to encourage them to stay and fight). However a sergeant may often be detailed to supervise a work-party that does not require an actual leader.
Soldiers work, fight, eat, and sleep together in squads. In additional to their personal equipment, the members of a squad will also each carry a share of the squad's equipment, and share duties in the squad. The seniormost veteran is generally the squad leader, who can be given the rank of corporal to signify this distinction. A squad generally forms two files in the formation. The leader of the second file may be given the rank of lance corporal. The term lead is also intentional, as the file leaders are generally the first rank of the company, since they are generally veterans.
A patrol is generally composed of half a troop that has been detached from the company for a specific purpose (such conducting an actual patrol), and is usually led by a lieutenant with the assistance of a sergeant, whilst the other sergeant remains behind supervising the rest of the troop. It is generally not capable of prolonged independent operation, and will be required to return to the company proper within a few days.
As suggested above, specialised squads may be attached to a company. For example all Roman centuries had an attached artillery squad with a single ballista. These specialised squads are more important for independent companies that are not part of a regiment (such as mercenary companies). For example most mercenary companies will employ a squad of mounted scouts to ensure that they are not marching into an ambush. Such an element will not generally take part in the actual battle (but will usually maintain security on the company's baggage train). Similarly a squad of skirmishers might deploy ahead of a company in battle, but would fall back when the company itself enters battle.
When functioning as part of a regiment these support squads may be gathered together on the battlefield as an ad hoc company. For example it is common to group all the ballista squads of a Roman legion together as a single company on the battlefield, because they are more effective that way. But in general the individual squads are considered part of the company and the responsibility of the captain commanding it. That said most regiments will have their own support companies that perform a similar role on the regimental level. For example a legion will have an attached artillery company of catapults, or a company of mounted scouts, for example. These may be attached temporarily to individual companies but will remain regimental assets under the actual command of their own captain.
Finally we shall go back and deal with the largest units.
The first thing you have to realise is that an army is an entirely temporary structure that only exists for the purposes of an active campaign. The composition and strength of an army will vary each campaign season, as determined by the royal marshal (the supreme commander of all military forces in the kingdom).
By default an army consists of six regiments. This restriction is based entirely on logistics, and may be affected by the general terrain of the border regions. For example the offensive army on a mountainous border might be restricted to a single regiment - whilst the defenders would be able to mobilise a full six regiments of defenders in response (which is why these borders might be considered quite safe from warfare unless the enemy manages to bypass them (usually by suborning the defenders) and ignoring this deployment restriction.
A Kingdom is generally only able to field a single army, whilst a High Kingdom might field two armies, and a Great Kingdom can manage three armies. That said, generally only one army will be mobilised in each campaign season (summer), both for logistics reasons and because only a fool will fight on two fronts. Lesser sovereign nations may be able to field smaller armies on campaign. For example a sovereign duke might be able to raise a regiment or two, whilst a sovereign prince might manage as many as four.
The royal marshal is the supreme commander of the military forces of the kingdom. They determine which regiments will take part in the campaign this season. The decision to go to war is of course a political decision (although it may be a political decision of the enemy king). In a feudal society the royal marshal is often the duke (the most powerful noble of the kingdom), whilst in a barbarian kingdom it will be the designated war chief. Note that the opportunity to go on campaign may provide plentiful opportunities for loot and martial recognition, so many regimental commanders will attempt to influence the marshal in his choice of regiments.
The marshal will select a general (which may well be themselves), who will be the over-all commander of the army. The general will be aided in this endeavour by the general staff. This will include two sub-commanders, who will be considered to be lieutenant (or major) generals. These will often be selected from a pool of appropriate commanders. Again, like the selection of regiments, being an active general on campaign (rather than being part of the military college at home) is a prestigious position and influence will be brought to bear on the marshal (and competence in these matters often takes second fiddle to social consequences). In a field battle (with the entire army) each of these sub-commanders will command a flank (with the most prestigious command being the right flank), whilst the commander commands the centre (generally as a result of communication restrictions). Note that these generals do not actually lead any troops but rather maintain their own headquarters element and simply direct the battle via messengers. Troops are generally only lead into battle by lowly field-grade officers (captain and below). When your general starts fighting, you know things are desperate.
However a full army is generally a big and cumbersome thing, especially when moving through hostile territory. This is especially true if the army has to rely on foraging, rather than carrying its own supplies with it (an army also eats a lot, so foraging and purchasing of local supplies is encouraged [and necessary] with many pre-modern armies). As a result an army will often have to divide into three separate divisions whilst moving (or when setting up camp). These divisions will remain in general contact via messengers, and come back together if it is necessary for the army to fight a field battle. Each of the aforementioned generals will command one of the divisions, with the mainguard actually being commanded by the full general. [Note that while the term mainguard is used here each division will almost always have to take a different route, otherwise the trailing division will only start marching when the lead division has already stopped to set up camp for the next night, slowing progress heavily.
If an army has any allied forces these will generally be specifically lead by their own commanding general and will typically function as independent units on the battlefield. Allied generals are always considered to have the rank of major general (which is the same level as a lieutenant general, but inferior in rank).
In addition the general staff will contain a number of brigadier generals (or brigadiers for short) They will generally command smaller detachments from the army, often on an ad-hoc basis. Officially most people consider a brigade to be the size of two regiments, but the truth is a brigade can be any size. It can even be composed of companies from several different regiments, especially if a particular troop type is required for a specific mission.
What all of this discussion is really leading up to.
Military Level 1 - Company Barracks [requires Small City]
A mayor, count, or provincial governor may add a barracks for a company to a city (with royal permission of course). These troops will technically serve as the town guard, but may also be quickly mobilised to deal with any troubles within the local region (such as internal or slave/peasant revolt). It may not be sent on campaign however.
Alternatively this improvement may represent the winter barracks and home base of a mercenary company (which should also be constructed with royal permission of course). Most of the year the barracks will just house the support elements of the company (including the recruit training elements), but will house the full company over winter. Whilst generally a private concern, the presence of a mercenary company will bring a nice income to the community (despite the added problems of mercenaries). Many veterans of the company will retire to the region, and the families of the soldiers will often be found here. Thus even if the mercenary company is a private concern, it will still have strong ties and loyalty to its home city.
Note that an individual small city can generally only support one or the other type of barracks. Royal cities prefer the first type; the so-called free cities often are a haven for the second type.
Military Level 2 - Regimental Barracks [requires Medium City]
A count or provincial governor may build a barracks for a regiment. This will be a regular regiment (either infantry or cavalry depending on the culture of the region). Whilst based in the designated city, the actual support and personnel of the company come from the entire county. In a feudal society this will generally represent the assembled forces and supplies from the region rather than a central physical location.
A march (border county) will always have a regimental barracks, even if the county seat is only a small city (it is assumed that the rest of the kingdom actively supports the existence of the regiment. These will always be standing forces (see the notes on frontier regiments below).
Military Level 3 - Guards Barracks [requires Large City]
The count of a rich county or a provincial governor may build a second regimental barracks. This will be a guards regiment (better equipped and higher-ranking than the other "regular" regiment). However the guards regiment will generally be filled by the "better" class of people and as such may have less actual direct combat experience (even when on campaign) than their regular cohorts.
Note that a march will always be home to at least one regimental barracks even if they are too poor to support one normally (in which case support derives from the kingdom as a whole). However the existence of the frontier regiment is why marches are generally larger than most - they also benefit heavily from tolls and tariffs on legal trade (and indirectly from smuggling of course).
This is the home of the Frontier Regiment that mans the defences of that frontier and responds to attacks from across the border. In addition to the static garrisons, the Frontier Regiment may be used for campaigns on that frontier, but for that frontier only. If a rich march has the opportunity to build a second regimental barracks, it will be for a regular regiment rather than a guards regiment. This regiment may be deployed normally, but it will often retain the border loyalties of their frontier regiment brethren.
The frontier regiments are almost always standing regiments and do not need to be mustered. They are always on duty guarding the frontier.
They often have a fierce loyalty to their local marquis, even if they a technically royal troops paid directly by the crown, and will often side with the marquis in a conflict with the crown. [Frontier regiments may march on the capital in the event of a civil war, leaving the border protected by just the local garrisons).]
Many frontier regiments are composed heavily of borderers who have a mutual antagonism with their opposite numbers across the border, thanks to generations of raids, skirmishing, and warfare. Whilst often under-equipped and irregular in nature, they may have more actual experience with combat than the other regiments of the kingdom. However, because of their relatively low status they also get the dirty and dangerous jobs on campaign. They are also more likely to commit atrocities against their traditional enemies.
Levies and Militia
Militia are considered to be garrison troops, and this generally outside the context of this discussion. Note that a company barracks improvement could be considered the barracks of a standing militia, which could then be deployed through the region (as normal for a company with a barracks). This is effectively what was done during the American War of Independence, before the formation of actual armies.
Levies are basically peasants and freemen that are conscripted into service. Whilst they are generally poor quality troops it is possible to raise a large number of them quite quickly, at the cost of doing substantial damage to the local economy (especially if they take excessive casualties - which they often will if they are on the losing side of a battle). As a result many countries have enacted laws that prevent nobles from directly raising the levy without the express royal command of the king. That said if a noble's estate is attacked, the peasantry will be expected to help defend it.
[Note that in a feudal society many freemen (the yeomanry) will actually owe military service as a part of their position, and will actually be counted as part of the feudal troops raised by the local regiment when it musters.]
This is a generic explanation and will almost certainly vary between different kingdoms. For example whilst the post-Marian Roman century is generally an excellent example of a company, the next level of organisation - the cohort - is probably best considered to be a battalion rather than a regiment. Whilst it does generally define the individual troop type (legionnaire or auxiliary) within a legion, it does not provide the logistical support normally associated with the regiments described above. Instead the organic level of independent logistical support is the legion, which is actually about the effective size of a brigade. Thus the legion is the equivalent of the regiment described above. However since these legions are twice the size of a normal regiment, an actual Roman army can only be composed of up to three legions. However as an empire, Rome is likely to be fighting wars on several frontiers at once, or dealing with internal problems (such as subject kingdoms deciding that they no longer wish to be part of the great Roman experience after all).
[Incidentally after the Julian reforms the centuries of the first cohort are of double size, which gives lie to the idea that the centuries are always a good match for the theoretical company.]