enemy getting stuck on siege towers - help?

[Caernarfon]

Hey I have a problem that maybe someone could help me with. (also I searched for this topic but couldn't find a thread)

I am playing as Brutii... and almost everytime an enemy faction besieges one of my cities, I end up with enemy units stuck(won't get off) on the seige tower bridge to the wall.

I can't kill those units from the wall so I always have to go outside once the enemy retreats and try to destroy the tower with my infantry.
(the first time this happened, I sent a unit which actually went up the tower and attacked the enemy on the platform -- I hasnt worked since)

Thats the other annoying thing.... it takes forever for the infantry to destroy those towers, expecially the largest ones. I have to sit around for 5-10 minutes while the units are messing around.

Does anyone know what causes this or how I could get them to stop getting stuck there? Or at least a way to destroy the siege tower a lot faster?