I see this exploit occurring of trapping keep attackers inside the keep by letting them destroy a wall or door then repairing a little and it quickly pops back up again trapping a small amount of attacking team inside to get slaughtered. This to me is an exploit, and very unrealistic and should be fixed.
How to fix this is simple:
Once an keep is under attack and a wall or door ends up in a state of being completely destroyed, it can not be rebuilt until the attack has stopped. Alternative way is you can put a time limit on completely destroyed walls or doors of say 5 minutes before it can be repaired again. One or the other should solve this unrealistic exploit.