Another thing that would fix the Zerg is if the doors to the keep were closed when an attacking faction got within a certain range. I find it absurd that you can be using a battering ram on a door and an engineer can hop out, drop a turret, and run back inside. If that door opens, it should open for everyone. This would also have to be true for the postern.

They also need to add a ramp to the Keep Lord. Concentrating an attacking force on one ramp, in addition to the issues caused with collision detection, make it exceedingly ridiculous to get a substantial force to deal with both the Keep Lord AND the defending Warband, who's numbers will likely be high with the addition of Guild-held keeps and the rewards you receive for retaining them.

Also, the keep lord. The need for a keep lord, or at least his four guards, is lessened as more players defend keeps. At some point, the strength of the Keep Lord and/or his entourage should be lessened the more players defend the keep.