In RTW, you can sally forth against seiges everytime you load your game.

so:

1.Get seiged
2.Sally
3.Save
4.Load
5.Repeat 2-4 until seiging army is defeated.

this works much much better with horsearcher units defending, as you will invariably lose no men while causing huge casualties on the enemies before you retreat into your settlement and quit with the successful sally, every time. and all this is done in the same turn.