I think what Siege really needs is multiple capture points, so it's more of a progression through the map rather than a race to the flag in the center/back of the castle. Theoretically defenders can just ignore everything in the map and stay near the flag and do better than if they were to actually try to use the walls and such. It's generally more effective, leaving the attackers with a relatively boring trek assuming there aren't people still herpy derpy on the walls.
So far the closest I've seen to what I'm looking for are maps with several gates leading straight for the flag, but you have to fight to get to each of the controls, so there's sort of a progression in that sense, and a greater incentive to protect the gate controls/walls rather than the flag.
Was this post relevant to ladders? A little, I suppose, just invisibly. >=o