The only towers I think are somewhat unbalanced are Sardon's and Castle, and possibly Deadwood. Sardon's because the position of the moving spikes and the room's layout make it near impossible to get to the flag room without being reduced to low health, and Castle simply because of the attacker respawn point and how long it takes to get back to the flag room. It can also be said that the defender respawn points for both of these towers make it a bit unbalanced due to the fact that in Sardon's, it's right in the flag room, while in Castle it's in the room before it, which is nearly empty most of the time due to reasons stated above.
A nice way to balance these two towers would be to change the layout of the spike room in Sardon's to make it less difficult to get through without dying or nearly dying, and to place the attacker respawn point somewhere inside the castle so they aren't delayed as much by the sheer distance and those guards that stand at the entrance that take almost two full seconds to let you in. The fountain is a good possible attacker respawn point, but it doesn't specifically have to be there.
And those are my thoughts/ideas/two cents.