Castles are the foundation of land ownership. A castle provides its owner with taxes from the province it is located in, as well as from garrisons in other provinces which are bound to the castle.
The owner of a castle automatically receives the remaining tax base from the castle's province at the end of each month. If a garrison is stationed in the same province as a castle, the garrison will first pay maintenance from the province's tax base, then the castle will collect whatever is left.
Each province may contain only one castle. The castle must be built in the outer province or in a city, if the province has one. (Tax revenue for the castle is the same no matter where it is built.) Castles may not be built inside other sublocations.
A castle alone is not sufficient to rule a province. A garrison must be stationed outside the castle in the province to protect it.