So in all my viewing I have not seen this unit in play even once!
Lets examine why…
Has little tactical benefit, and is better to just take out walls and once long range siege comes into play is worthless.
How to make it better.
I think we were all expecting and hoping for some more on wall fights. From what I have seen this is partially impaired because units scaling downed wall sections cant attack upwards if blocked from getting to the top.
So the siege tower should be incredibly tanky. I am not sure if units on walls can use their flame attack on them, but since the machine would be covered in fire retardant skins, should be resistant to units on walls using torches. It is something hand units should have to melee attacks on the base to quickly destroy, like a unit rather than a building.
it should stroll up to the wall, and then drop its bridge, and any units under the bridge, should get hurt or killed. this would then make some room for the garrisoned units to get onto the wall. and cause the defender to micro away from them.
Each tower while garrisoned should get up to five archers and one for every 2 untis garrisoned. That means it would take atleast 10 units to garrison in the unit to fire all five shots. 1 unit would not give any archer shots, The range would be shorter than most archers and any units on the wall would not get their defensive bonus for archer shots from the tower. However since the tower would be pretty resilient to archer fire the advantage would go to the attacker. then once in place the tower would act as a path way for units, so no longer would you have to garrison, or ungarrison them, units would just find a path from the ground to the tower to the walls to the other side (using the enemy gates)
Any unit near an enemy gate tower should then make the gate open for the rest of the army to get through.
Another thought for a special unit is to have one with a small springald at the top, that would be less range and less damage(but faster ROF) than a normal one, but since it would be at tower height could shoot over the walls which other springalds would be vulnerable. (trebs and mangonels could attack them though)
Will it even be enough? maybe not… but could make for some fun diversity and niche scenarios. Right now it seems to have no viable use. But if it were easier to control the high ground advantage that walls gave then people may want to fight to get that advantage using enemy walls.