Question about seiges and siege equipment

[Scotsmanforlife]

I'm attacking the russian city Vilnius and other than the usual cavalry and foot soldiers i have 4 catapults. Now, this just happened for the second time so it's not a fluke.
Here's what happened: I assaulted the walls with my 4 pults and figured when they ran out of ammo i would send in the rest of my troops. Well, as soon as the ammo ran out i got declared the loser and the fight was over. At first i thought something else had happened to make me lose. So i attacked again and used the pults till they again ran out of ammo with the same result of me losing the fight.
Now here is my question, why does running out of ammo make a person lose? Or is it because i don't attack with my other troops? Thanks for any replies.

[Warluster]

Because there is no more siege equipment the game automatically thinks 'You can not get in' so you lose

[Scotsmanforlife]

Ah, well that makes sense. Thank you very much!

[katank]

Use them to smash open the gate or breach a wall first before tasking them to hit other things and you should be fine.

[PureFodder]

On a multi-walled Fortress/citadel don't ever fire your last artillery shot. Despite the fact that your infantry can get into the inner walls by the connected wall network and despite the fact that I had already made a hole in the inner wall so I could get to the central square as the last shot fired, I get told that I just lost......