when should you build a new fortress?

[Meglobob]

You should build a second fortress as soon as possible, so you can recruit another mage per turn if nothing else.

The 3rd/4th and even 5th should be built as quickly as possible too, for exactly the same reason.

After that, I personally just place my forts to defend strategic locations and very important gem sites only. I prefer to capture fortresses from my opponents from then on.