While the idea of putting troops behind walls is both historical and interesting, in the context of current gameplay it is not nearly as efficient as simply breaking the walls with catapults or rams, however there is the other usage of siege towers which is fire superiority, being able to overlook enemy fortifications and essentially clear them or support an assault on them. It is not like we can map an exact historical battlefield anyway. We do not build siege towers on site and then roll them up to the walls, and we also roll them over terrains they historically cannot go over, their very movement is largely ahistorical, so frankly I say add interesting features with the understanding that yes players will probably use it as an infantry carrier/tank or something equally amusing, but that is ok