[Turin]

I would add the fortress to the usable castles. It´s almost as good as the castle, but costs 20 design points less.

I agree that the others are pretty useless. So how much should they cost, so that you would choose them?

I would see every castle as viable, if these changes to design point costs were made:
watchtower,mausoleum,fortress,castle,wizardtower unchanged.
hillfortress: -20 points
fortified city: 40 points
citadel: 40 points
dark citadel: -40 points
mountain citadel: -60 points

I´d probably still go with watchtowers in most games, but I might choose the negative point castles if you just need those few extra design points.
The citadel or fortified city would be a real alternative to the castle/fortress, if you want to have an high admin castles.