(If this has already been covered in another thread, please ignore this.)

Here's another thought:
In Cyrodiil, when you battle for a fort or something, you use catapults or trebuchets to destroy walls and gates to storm in and capture it. Then what...? The walls that you just destroyed with your attacks are nothing but rubble. Do those walls simply re-spawn with the magical wall summoning spell, or do they remain destroyed till you, as the new tenants, fix and re-build them?

I personally would LOVE it if they added this bit of realism. As an attacking force, you may not want to tear down a wall as you, as a whole guild, decide that it the reward of an easy capture is out weighed by the time and resources it would take to build the wall back up.
Its easy to replace a broken gate, but an entire section of wall!?

Or maybe you all decide "Lets tear their fort to pieces! If we lose the fort to someone else, then they can deal with the rebuild."
It would add another aspect to attacking. Do you want to go in guns blazing and take the time to clean up your mess, or take the time to plan an attack and not have to repair half the fort?

The alliance that attacks and captures the keep must rebuild the walls/doors themselves, they don't respawn. Whatever you break, you buy.