[Semigall]

As i'm quite new to the game, could you please give me some hints about fortress building ?

Where & when to build, etc.

Because, as i understand from manual and other information, fortresses are expansive and are built in more than one turn, so i can't just build them everywhere...

Yet i still don't really understand the supply system in the game, so i'm not sure what are the best places for fortresses.

Some map's chokepoints maybe ? Or some resource rich provinces ? Or territories with some natural defence ?