The reason to take walls rather than destroy them :)

[Slaists]

The other night I was assaulting a greek city (with Brutii), the city had a decent garrison of cavalry and armored hoplites but I outnumbered them vastly. The problem came in the form of a greek relief force approaching the city from the North side while I was storming from the West. With the relief force, the greeks outnumbered me two to one. Once I had the West side taken by my siege towers, the walls started shooting at the approaching greek forces and routed/killed them all (helped by my wardogs) before they could join in the fight in any serious manner. I think, even their general (an heir) bit the dust pierced by an arrow from the wall... I lost one seige tower (out of three) to arrows from the walls.