In the last session of the Dolmenwood game I’m running using The Black Hack 2e, I ended up improvising some mechanics for a siege, and it went pretty good.
Briefly: the building has HP, and so does the horde outside. Each round PCs can do stuff themselves and order groups of underlings. Attacking the horde whittles it down, and PCs and underlings can shore up the buildings HP, while the horde attacks each turn, lowering the buildings HP. It went pretty well!
I have more details in this post I made: Penny Ventures: How To Survive A Zombie Attack: Improvised Seige Mechanics