The map is pretty first of all.
When I played it there were like 5 HRE on, which could break through pretty easily, they won quite a few rounds while not being that many. I have played it once, but it seemed reasonably balanced then.
Yes the tower is a huge chokepoint, but there is atleast three ways to get to the top part of the keep. What you should not forget is that defenders do not at all always spawn on top. I have been spawned at the wall, which leaves me to find a staircase down, find a staircase up, walk all the way up and then I can defend the chokepoints. I think this balances the map, cause in the end there is not always that much defenders on top of the tower.
Although the far away spawning for defenders saves the balance, I hate it. In any siege map, I think the defenders should spawn reasonably close to the flag, so they can defend it. I hate those retarded castles where I spawn outside the castle as defender.
I will wait till I play it some more with a full server before I vote