On a serious note, an actual Seige Tower would be very cool. Make it take 30 seconds to assemble on the field. Activating the door at the base transports you to the top of the structure, but the door operates on a one person at a time function like activating a chest. This would give a legit stratigic option for getting a force through a keep's outer wall. It could also be set up anywhere just to be used as a perch for ranged attackers.
but giving the ability to potentially bypass the outer wall and flag the keep by getting the inner down to 50% would be stratigically interesting. This was in effect to a degree when people could gap close over walls, which sucked because it was an exploit, but a legit means of bypassing the outer wall is warrented, I think.