KingdomsA short discussion of what I am referring to when I later start talking seriously about Kingdoms, Principalities, and Duchies. And I apologise in advance to any historians for the many assumptions an inaccuracies in the examples I am using to illustrate these terms.
The Kingdom is the basic polity of this game (regardless of whether or not it is actually ruled by a monarch or by some other method). It represents a specific people (a collection of tribes), and their sovereign ruler (however that sovereign might be selected).
The ruler of a Kingdom is a King (12th Level Noble).
The physical extent of a Kingdom is probably much less than you expect. The map of the British Isles shows a rough approximation of the Kingdoms that were historically located in the island. Note that boundaries were fluid through time as kingdoms warred against each other and conquered territory (this snap-shot is about 600-700AD), although at this time Northumbria has occupied the eastern half of Strathclyde (and will eventually occupy Lindsey, prior to the Danish invasion that reduces it to a shadow of it's former self). Similarly Mercia and Wessex have been shown to have swallowed the technically Minor Kingdoms of The Twicce, Middlesex, Sussex, and Kent (as they would eventually do). In Roman times Lindsey and East Anglia would have been considered part of Mercia (or Flavia Cesariensis as the province was actually called). In fact East Anglia's prominence here is probably as a result of the later Danish invasion (which aslso swallowed Essex).
Ignoring Ireland for the moment, the important Kingdoms here are Wessex (Saxon), Mercia (Angle), Northumbria (Angle), and Alba (Pict [later Gael]), and which should be considered exemplars of what it is to be a Kingdom.
Lesser Kingdoms and Grand DuchiesIf a people do not have a sovereign then they might be considered a Lesser Kingdom. This is the case of Wales, inhabited by the Welsh people, who may respond as a group when invaded from outside, but in reality are composed of a number of Minor Kingdoms led by sovereigns who are effectively Warlords/Dukes (10th level Nobles). In the case of Wales the Minor Kingdoms are Gwynned, Powys, Seisyllwg, Brycheiniog, Gwent, and Dyfed.
Alternatively if the sovereign rules little more than a City State they might be considered a Prince (11th Level Noble) ruling a Principality (which is considered the equivalent of a Lesser Kingdom). This is the case with Lindsey, which is centred around the old Roman city of Lincoln.
A Duchy is considered the smallest sovereign polity. The ruler of a Duchy is generally considered a Warlord/Duke (10th Level Noble), and may be tribal in nature. Cornwall is just such a Duchy, and home of the Dumnonia tribe from Roman/Celtic times.
Sunderland (and the Out Isles) is considered to be the home of Norse immigrants, but generally lacks any overall sovereignty (most of the settlements are effectively autonomous and therefore under the control of a Baron (7th Level Noble) at most. Caithness would as a result be considered a Norse Minor Kingdom (equivalent of a Duchy).
Strathclyde is home to a polyglot of peoples from the Gaels to the West and the Britons to East, tahnks torepeated invasions from the North, West, and South. Technically Northumbria at this time extends through the western half of Strathclyde, having conquered the Minor Kingdom of Beornice, although the map shown limits Northumbria's Northern reach to Hadrian's Wall (after the Danish invasion the Kingdom retreats completely to the North as a Lesser Kingdom). That's the reason for the dotted line. This is the infamous Border region which really wasn't under the control of anyone for very long. So it's mostly squabbling Barons (7th level). Eventually, with greater order established over the region it is controlled on both sides by the March Wardens/Marquis (9th Level), drawing on troops of the respective High Kingdoms.
An Empire will often consider Kingdoms to be the basis of their Provinces (which are ruled by a Governor). For example the Roman Empire considered Northumbria to the the province of Maxima Cesariensis, Mercia (along with East Anglia and Lindsey) to be the the province of Flavia Cesariensis, Wales to be Britannia Secunda, and Wessex to be Britannia Prima (give or take some real estate).
A Kingdom technically consists of a single People ruled by a single sovereign. A High Kingdom is formed when multiple Peoples (or Kingdoms) gather together in a single High Kingdom.
The ruler of a High Kingdom is a High King (13th Level Noble).
When the King of Wessex reconquered the lands taken by the Danish invaders he became the High King of England (an Anglo-Saxon High Kingdom).
When the King of Alba (or as it was originally known, Pictland) conquered the surrounding peoples (the Norse of Sunderland/Caithness and the Gaels and Britons of Strathclyde) he became the High King of Scotland (which, one must admit, was simply another name for Alba).
In both of these cases the High King is the absolute monarch of the lands under their control.
Ireland also has a High King (13th Level Noble), but is not a High Kingdom in and of itself. Instead one of the subject Kings is acknowledged High King and becomes suzerain over all of Ireland. He receives tribute from the other Kings, which explains his heightened precedence, but his personal authority and power is limited to his own Kingdom (plus the tribute). This is the smallest form of Empire, and thus a High King may also be referred to as a King of Kings.
Great KingdomsThe Great Kingdom is the largest polity where a sovereign may exercise direct rule over the kingdom, although they only can do so with the creation of a full-fledged Royal Bureaucracy through which they delegate royal powers.
The ruler of a Great Kingdom is a Great King (14th Level Noble).
The United Kingdom of Great Britain and Northern Ireland is the classic example of a Great Kingdom (actually Great Britain alone is sufficient).
I suspect that most of what people think of as independent kingdoms are actually Great Kingdoms - Burgundy, Occitan, Catalonia, Portugal, and Japan are all examples of Great Kingdoms.
Great Kingdoms are very much an intermediate state between the idea of a Kingdom (with direct rule by the sovereign) and that of an Empire (with indirect rule by the sovereign). With an Empire the Royal Bureaucracy is converted into between 3 to 8 full-fledged Imperial Ministries that essentially run the Empire on the Emperor's behalf. Each Province/Kingdom is directly administered by a Governor (which tends to reduce the internal tensions - at least those arising from the populace - within the Empire).
The term Kingdom is a term of art (as is King, Prince, and Duke). There are many Kingdoms in the game that are republics, meritocracies, theocracies, plutocracies, timocracies or even those whose sovereigns are women (and thus should properly be referred to Queen, Princess, and Duchess). But the idea of the basic Kingdom is that they are a single People (which may be composed of one or more Tribes that identify with a common cultural heritage), that is ruled over by some sovereign authority (King, Queen, President, Governor, etc).