[Twan]

Another option would be to raise the tax bonus from admin (admin value instead of admin value/2 %, so fortified cities would be really interesting to build on rich farmlands).

It wouldn't solve the problem of hard to build forts with low admin and hard defense.

To give them an utility, I would find logical if supply availables for besiegers were reduced a lot instead of increased by the presence of ennemy forts. When provinces are attacked, peasants should logically take refuge in the fortress, especially if it looks hard to take, instead of helping the invaders, giving them food and gold. Diminishing ennemy supplies and province income by base defense value of the fort /10 (so -70% on a province with mountain fortress) would be a good idea IMO (but may advantage too much undead or nosupply units nations).