I had a lot of fun with siege ladders. But I have to admit that many maps are much better balanced without them. I think the main problem with siege ladders was:
a) ladder bridges straight to flag
b) a win ladder where half of attacker team climbs up in 10 seconds and rushes the flag
I think both could be fixed by introducing carry weight based on steepness of the ladder. If the ladder is very steep, it will carry the most men at the same time, but they will crawl up like snails. If the ladder is very flat, two men on it at the same time already break it.