[Edi]

The types of undead generated are random. You will get Velites, Legionnaires and Knights of the Unholy Sepulchre from provinces with forts. The Knights also require a temple, I think. The precise composition you get in each province is up to chance.

It doesn't matter what kind of fort you have, so long as you have one, so tearing down forts is not recommmended. They cost too much money.