I apologize in advance if I sound like a mad man, I am going to playtest this a bit tonight but I'm positive it will work just fine.
make a basic terrain piece as the street level of a city, including fences and gardens and fountains and all sorts of stuff, infantry and machines can enter only. Make sure the streets are a proper width for the bases to fit and all intersections are chamfered to help out with fitting the bases as well and make for easy game play. use the road rules to help them move but keep in mind that irregular formations cut your movement to half pace
Fighting street by street, you will be formed up long ways but just like in fortifications and on top of walls, no flanking bonus' and no support bonus' for either side and no "in the open" bonus'. Earthworks can be purchased to gain defended status bonus'.
Finally, back to the buildings....make it so the buildings are removable but not for your troops, only because they will have structure points like a fortification wall section or siege tower. now you have a defender burrowed into the city like a tick that you need to get out to protect your flank so just start destroying buildings until the right ones are gone and then they no longer get the terrain bonuses! Calvary and anything else can go and get them. Maybe allow infantry to destroy buildings, auto hits like a battering ram, defender can purchase booby traps for the buildings like burning oil or rock droppers in a siege game.
With the individual buildings getting destroyed, think of the scenarios possible, save the city from the orcs!!!! keep at least 2 buildings in tact. etc. etc.