Military forces can capture a province they are in, taking control of its settlement or trading post. Control of a province is usually necessary to allow negotiation for it in a peace deal; thus, control is a precondition of ownership.
Capturing Unfortified Provinces
Trading posts and unfortified settlements are captured immediately whenever an unopposed and non-retreating enemy military unit is in their province. This includes naval forces, which are allowed to enter unfortified enemy ports if they have military access. Furthermore, because land forces do not interact with naval forces, naval forces can capture unfortified provinces even when enemy land forces are present. Strange but true, and highly exploitable.
Because capture of unfortified provinces happens immediately, usually there is only one country's forces involved. However, in the rare situation where two countries' forces are both in a province at the same time when capture happens, the priority order for capture works like it does for siege control.
Capturing Fortified Cities
Fortified cities must be sieged or assaulted to be captured. If the sieging force gets a "capture" result (which usually requires at least several months of sieging, and often much longer), then the country which was controlling the siege at the moment of capture takes control of the city. Similarly, when defenders' morale breaks or the last defender is killed in an assault, the country controlling the siege takes control of the city.
There are two exceptions to the rule requiring siege/assault to capture a fort, both of which involve rebels. Rarely, when a rebellion is triggered, the rebels will capture the fort immediately. (This effect models the garrison rebelling.) The chance of immediate fort capture happening is increased by high net revolt risk in the city, and decreased by the level of fortification. The other exception is in a civil war, where there is a 33% chance, per province, that it immediately becomes rebel-controlled.
Fortified cities cannot be captured by naval forces.
Effects of Capture
The primary effect of capturing a city is that control of the city passes to the country whose forces captured it. Also, when a non-pagan country's capital is captured, the country controlling the capture gains the loser's maps.
There are two minor effects that may occur with the capture of a fort: