Actually, I ended up finding it in the most unlikely place: The 'models' folder. I thought this folder would just hold the 3D objects so I ignored it until now. But the file is called 'MODELDAMAGE.TXT' and all you have to do is go to the gatehouse and erase the line that says 'attackablewithhandweapons'. Now you can't conduct a siege on a stone castle without the proper equipment. ;)