How many fortresses to build?

[Etaoin Shrdlu]

Might it make sense to throw a few site-search spells at the proposed fortress province? Just in case there's one hiding next door already? It would be mighty annoying to discover you've built a castle and find that the Crystal Citadel or some other high-defense fort was just waiting for someone to find it...
Failing that, I'd put forts:
1. On high-resource provinces (especially if I'm using heavy forces still)
This makes it easy to reinforce chokepoints, even if the castle is not on the chokepoint per se...
2. Places with hard-to-build units (why oh why are Knights only available in resource-poor lands?)
3. Where I was going to need a lab anyway (but for interesting mage types only. If there's three Libraries within range I'd likely put one in the middle and pull the Sages into the castle from next door...)

[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.