Why build Fortresses?

[Endoperez]

I thought it might be that.

You won't get any Niefel Giants, as those are only available from the capital. Of the other troops:
Jotun Javelinists are nice. Strength of over 20 gives their Javelins wonderful range, and as they are one of the few ranged weapons with which strength is added to damage, they also deal wonderful damage...

Also, some kind of shortbowmen are nice to have. If you have conquered lands from a Deer, Wolf or other tribe, you can also recruit their Shamans if you build a temple and a lab. Unless you have negative magic scale, they are more cost-effective researchers than your giants.