At the moment, many strongholds are placed in meaningless places. Like under mountains or middle of a desert, or at the end of a map. Many armies can simply go around those fortresses without losing too much of their time. Why attack strongholds when you can attack cities...
I think fortresses should be placed across a bridge or more strategic points where people are forced to attack it or they have to go around much longer distances.
If moving strongholds can't be done, maybe there should be some other things to compensate for it?
