I've seen a lot of defensive capabilities.
What are the offensive perks from a fortress? I would assume you research them in the library? It would be cool to see things like 10% attack to cavalry or maybe even a new offensive unit for fortresses? Upgrade swordsman to knights maybe?
What if fortresses had more uses for copper? Like if you have a fortress that has troops supporting another castle, you can send copper to that castle and those supporting troops will proceed to another castle as support. You wouldn't have to recall them home first then send them back out.
I'm just spitballin fellas