I don't know ..doing siegey stuff like digging trenches and throwing up berms as they seal off the city or get closer to the walls.Just in case they make an assault.

I know,the game doesn't simulate that.

Wear and tear from the siege.Accidents,illness,injury,the occasional raid to test the sieging army or probe for a weak spot.It just seems odd that a sieging army never losses even a single man since the biesieged losses are not from combat.