There's a link to such a concept (for CK3) in my sig, which I'll copy here:
There’s a lot wrong with the “Holding” system of world construction in Crusader Kings 2. The way its tied to government type results in numerous problems, such as the Byzantine Empire turning tribal and being unable to become feudal, kingdoms randomly turning into theocracies and an ahistoric inability of the King of France to hold Paris himself. Furthermore, the system of “spend money to both make money and permanently have more levy” causes areas to always progress unless attacked by nomads and causes easy minmaxing with no-brainer building choices. It also enhances blobbing as there’s no disadvantage to having lots of army buildings in the capital – in reality, a large rich empire would be constantly spending enormous quantities of money to keep thousands of armed men happy, whether they were in fortified positions or not.
My proposal is very radical, but don’t worry; the POPs have nowhere near Victoria level of detail
[1]. Each would inhabit a province, while having a religion, culture, agitation, number of people and way of life. Agitation is very similar to revolt risk, while way of life does not correspond exactly to any mechanic in Crusader Kings 2 but is most closely linked to government type.
There are four possible ways of life: nomadic, tribal
[2], settled and urban. Nomadic is the way of life in the steppes of central Asia and the interior of Arabia, among others. A majority of a nomadic POP can be raised by a strong ruler to wage war with relative ease, but such POPs only give the tiniest trickle of income. Tribal is the way of life of the Daylamites in the Elburz, Albanians and Montenegrins in the Balkans, and the Scottish Highlanders, among others. Most tribal rulers can raise a large minority of their POPs in defence, and the most effective can raise similar numbers on the offensive, but tribal POPs only give a small amount of income. Both nomadic and tribal armies are strongly inclined to settle in newly conquered areas, the former particularly. Settled POPs, which cover most of the map at the latter start dates, give a moderate amount of income, but only a fairly small fraction can be used for defensive wars, and less for offensive ones. Urban POPs, such as the inhabitants of Bagdad or York, give the highest incomes to their rulers, but only a few percent can be used for external conquest.
Urban POPs are very desirable to have due to their great wealth but are hard to keep intact. Provinces have a POP “cap” which, as the POPs in the province approach it in numerical size, their growth will decelerate. The cap is increased multiplicatively based on way of life and the size of the area controlled from it and hence it can draw resources from. In rare circumstances, the POP size will significantly succeed the cap, due to for instance an abnormally long period of peace without plague, abnormal amounts of wealth flowing in through trade or being the capital of a large area
[3], in which case the extra population will split off to form a new POP
[4]; an urban one. Urban pops are particularly vulnerable to disease, sacks and general shrinkage. Urban POPs are inclined to become republics in times where their liege’s control is weak, and in select cultures, they may be sold town charters to become republics to grant the ruler who sold it an immediate cash boost. In order to protect an urban POP from war, it may be a good idea to build city walls, which leads into the structures feature.
First, city walls prevent any enemy army which just moves into the province from damaging the POP. Instead, to access it, they must besiege the city. City walls are also unique in that they can be built around other structures, like castles, granting additional protection. The disadvantage to city walls? They, like all structures, require maintenance. Zero or insufficient maintenance will cause structures to gradually decay, losing defensive advantage at first very quickly but at ever slowing rates
[5]. However, a half-decayed structure will be much cheaper to repair than building a structure from scratch.
There would be two non-defensive kinds of structures: palaces and temples. The former would be things like the Apostolic Palace or Palace of Westminster, while the latter would include Aachen Cathedral or the Al-Azhar Mosque. Like other structures, they would require maintenance, but Palaces would give more prestige the larger and better maintained they were, and Temples would give piety
[6]. Note that theocrats would not live in temples, but castles or palaces.
[1] They have a closer resemblance to EU:Rome or MEIOU & Taxes 2.0 POPs
[2] Not to be confused with tribal government type, which are actually much closer to chiefdoms.
[3] This would be done with a formula something like T=B*C+C/x, where T is the new total the province’s POPs are geared to (collectively) grow towards, B is the base POP cap of the province, and C is the total caps of all the areas controlled (minus perhaps some sort of autonomy modifier like Crown Authority in CK2).
[4] Or POPs, if the province is religiously or culturally diverse
[5] For instance, not maintaining a castle for a decade with substantially reduce the defensive advantage, but a castle neglected for 50 years is not much worse than one neglected for 25
[6] Both should be subject to diminishing returns, as someone who owns 100 palaces is not twice as prestigious as one who owns 50. (Indeed, this should be a general principle as it would stop prestige becoming a bottomless pool for major emperors.)
