Siege Tower Glitch - Enemy AI won't climb the walls and attack

[Hugo Stiglitz]

I was so excited to play my first siege defense with these epic city maps and as soon as the enemy got to my walls they just stood there doing nothing, all they had was siege towers so there was battle at the gates and I did not send troops out beforehand to cause a glitch. What is the deal?

[Drunk Rambo {The Dirty Dozen}]

Ah, I had a similar "problem".
My capitol city of the Sudreyar's on the Western Isles, Northern Scotland, was invaded by 4 stacked Viking units. 3 by sea and the other via land.
They forgot to bring siege equipment :)
I massacred around 2000 of them as they just marched around my walls. I ran out of arrows and javelins and let my NPC towers kill many of them. Unfortunately I had the battle timer on so it was a draw.
This happened twice. I was eventually able to go out and meet them on the field and wipe them out for good as they were heavily weakened via the sieges.