About the indecisiveness, thats the AI and sadly not much can be done except making less point of access toward the players which I wouldn't do.
For the ladders, I would either delete them (and make sure players cant jump off the walls, except the side that goes inside the castle) or maybe try to delete the AI mesh below the ladder, so they won't
circle. Making the mesh smaller, might get that % of them getting up bigger.
For the cavalry, I would suggest deleting the ladder, or making the path smaller, so the AI might think it's not possible for a horse to pass trough.
Why they wouldn't use the siege tower, the only reason I can imagine, is that they think the gateway or ladder is faster. Have you tried with a player standing on the wall at the siege tower, or maybe inside
the siege tower?