Building fortresses

[Crafty]

Wow, good comments. A lot of things might seem obvious to you, but not to me..

Here's a scenario, you are playing a double blessed strategy, leading an early charge. After clearing the provinces around your capital, you strike out, and very soon you run into a fort build by the AI. It looks poorly defended (the main enemy force is probably raiding elsewhere), you can probably beat it, but it will take a few turns to break the walls which is time you cannot waste in the early expanding stage. Or perhaps you should ignore it, and just take the other equally poorly defended enemy provinces.

What do you do? I always try to take the fort, which I think is a bad idea on big maps...