understanding a sally

[Gaius Scribonius Curio]

In terms of sallies by a besieged foe, does this mean that you have to force them back inside their walls to get a draw? I ask because I'm playing without a battle time limit as the Saka, and had a general and two low-end horse archers units besieging Taxashila, basicly halved the enemy forces without loss, but then was forced to retreat because I couldn't force their remaining forces back inside the walls. I'll return in six months to finish the job, but I just wanted this clarified.

Thanks,

Curio