Also, take into account that realms are managed in a very abstract form. Realms can be "landed" (own provinces, like nobles) or "non landed" (do not own provinces, like guilders).
A province has a certain terrain, which determines:
- Maximum population level
- Maximum magic level
The population of a province can be increased using actions ("Rule Province"), while the Magic Level of a province is always: Maximum Magic Level - Current Population Level. In Birthright population destroyes the nature places that are associated with power (except elves, they are an exception to this rule as they know how to live in harmony with nature).
Apart from population and magic levels, provinces can have 4 types of "Holdings":
- Law holdings: they represent the control over the province population. Usually the owner of the province is also the owner of most Law Holdings. They allow you to set taxes over other holdings.
- Guilds: represent commerce and allow to create trade routes.
- Temples: represent the faith of the people, allow to cast divine spells and influence the population loyalty.
- Sources: represent the magic power, allow to cast arcane spells.
Holdings are given levels too, so for example a Province (6) (level 6 population), can't have a Law Holding bigger than level 6 (Law (6)). The conflict in Birthright arises as several people can share holdings of the same type in the same province.
For example, a Province (4), could have two nobles, one the rightful king with a Law (3), and a rebel champion (like Robin Hood for example), with a holding Law (1). That means that in some parts of the province, the people think the rebel is the rightful leader and follow him. With 3 vs 1, the king shouldn't be worried, but there are actions ("Contest") that allow you to reduce other people holdings and increase yours. That's how conflict arises in Birthright.
And that's more or less how provinces are defined, no buildings, cities or other low level stuff, just 6 numbers and done (there are some extra things: fortifications, armies, trade routes,... but really, everything works around holdings and the conflict to own them or setup new ones in your enemies provinces).