The "town square" while taking cities

[Revelation]

Historically, once the gate was open or a breach made in the walls, that was it, usually. I think once your guys set one sandaled foot inside it should be Game Over.

Not necessarily. One sandaled foot inside? No thanks. Once a wall/gate is breached, then ensues the bloody street fighting, where morals, formations and tactics basically go out the window. It's a fight to stay alive!
It's all part of the game.
Who's to say it's not part of an enemy tactic to allow the walls to fall and lull the enemy in, then WHACK!
As was mentioned earlier, turn off the battle timer or take the walls quickly.
I would prefer the no battle timer approach, as planning and prolonged sieges can be fun. I find a phalanx of Hoplite mercs to be most effective in the city streets. Have them march through slowly with peltasts close behind a perhaps a unit of Roman heavy infantry bringing up the rear.
Let the games begin!

[Oaty]

Revelation you've got a point there

Hanibal was lured into assaulting a city only to find that there was an ambush setup inside the city walls. He rallied his routing troops outside the city and left with his army in tact but apparently foiled any further plans to take the city

[pezhetairoi]

I remember how Philip II assaulted Perinthus but over the course of the assault discovered to his great annoyance that successively the Perinthiacs had built 6 layers of walls inside his original breach to forestall him. He got annoyed and gave up. Oh, but for the day we can do that...

[sapi]

I agree with the 'no timer' stance.