Bleeding a sally/city

[d'Arthez]

Just play without battle time limit, and wait until the timer is up. Time-up draws do not allow for healing of casualties.

Cheap exploit of RTW engine:
You can try and let them ALL come out. Rout a single unit and take control of the walls. Simply lure the non-routing units away from the gates, and chase the routers with 1 or 2 units of cavalry. Extremely unbalanced exploit though (you can conquer mainland Italy with SPQR in 4 turns with your starting armies that way (!).

Frontline, you can also use the perfect_spy script, so you don't have to bother with spies to see the effect of your tactics on oppostion numbers.