very confused

ok,this is a first for me in any total war game.
playing as kislev,got to scripted arrival of chaos stacks around turn 10.
2 of said stacks lay siege to my capital and assault.i have only 7 units in the city.im doomed thinks i.
enemy destroying my walls with artillery when up comes the box saying ive won!
i return to strat map to see both stacks drop dead,i havent so much as loosed an arrow!
ive played countless mods for tw games and this is a new one on me.must be a bug,yes? cheers

Was the enemy's artillery able to punch a hole in your walls (as in 100% damage)?

yep,walls were down and i was waiting for the assault.cheers

That's odd. I could see if they weren't destroyed, but this is something I've never seen before.

why if the wall were not destroyed? the only thing ive had even vaguely comperable was being sieged by orks and they run out of ammo for their seige engines before they manage to make a breech and then they all stand around like lemons till i attack them,but i didnt get an instant win. chaos had my walls down in 3 places in about 5 mins real time and i was going to lose! cheers

Well see, if the attacker was unable to punch a hole in the wall (and you did not sally out), the defender automatically wins.

so,if the walls arnt breeched and i stay put i will automatically win? i didnt know that!
is there some sort of timer attatched to this? cheers

If you sally out (attack your besiegers on the campaign map instead of waiting until they assault the city) and the besiegers use up all of their ammunition, they will remain on the battlefield (sorry, I wasn't clear in my last post), and you will have to attack them.

However, if they assault your city (as in they are the ones that attacked on the campaign map), and they are unable to penetrate your defense before all of their siege equipment is either a) destroyed or b) runs out of ammunition, then you will automatically win.