The A.I. has 1 hell of a problem assaulting cities mainly due to bottlenecks. What I was wanting to do was remove all buildings such as houses and decorations and only leaving the buildings you build in there, such as the barracks stables and shrines. If the A.I. still has trouble maybe removing all buildings.

So is it possible to mod a town to only have a town square and walls and will it work on th campaign map.

This would emphasize defending the walls as it historically was. Since once troops got inside morale dropped, entrenchments/fortified positions would start popping up and usuallly it was a matter of days until the defenders and populace started getting raped enslaved and executed.l