I have a few idea of what you can do to improve your fortress:
- Garrison
Basically the men who serve at your fort, you can get them better armour, weapons, living space, trainers, commanders, etc.
- Defenses
Things like the strength of your walls, archer towers, barricades, drawbridges, etc.
- Surrounding countryside
You can improve the farms of the countryside, make gaurd patrols, improve roads,etc.
-Defending from attackers
Depending on your character's level your fortress will get attacked
lower levels you get attacked by bandits, then at higher levels you get attacked by higher level enemies, like necromancers, vampires, and sometimes draugr.
Also level dependant dragons attack, ranging in numbers that are either 1 or greater.