The problem of fort types

[Ironhawk]

Twan said:
A fort allow the defender to be sure to be attacked only one time per turn, so it essencially make him invincible as long he has a good number of powerful mages inside (and domes/resist gear to protect them from rituals) and the attacker don't have a sufficient number of assassins (only way to make mages use their gems before storming) nor teams of SCs able to take the fort alone.

Eh? Just because the defender can act first does not make them invincible by any means. If you have the defender seiged in a fort then they are at your mercy. Let them starve while you carefully prepare your army with whatever resistances you need to meet thier mages. Then just storm and wipe them out. I think you are seriously overrating the defenders ability to act first - either on the field or in a seige.