I read before that in siege autocalc treats it as a field battle, but this is jfor me really an example for how screwed it is. I was sieged in antioch (huge city with ballista towers) by about 1500 egyptians (mostly spearmen). My army was 3 missile units, several spear and a few bodyguards.
Just for the heck of it I decided to press start battle without changing anything and letting it run while I went to take a shower. Turns into a heroic victory - 54 losses. I load, autocalc: a clear defeat with over 600 losses.





In the last 3 campaigns I don't believe I performed a single (attacking) siege in a real battle as autocalc always does the trick.