How many fortresses to build?

[Xietor]

In Dom II i built more castles than dom III. Now that the Ai builds castles, i typically just take theirs and use my gold elsewhere: Pans are expensive, as are most of the Pangaea's troops. So I would rather just take castles away from my enemies.

I will build a castle at a clear choke point, as others have pointed out, and on squares that make rare troops that i really like.