Siege question

Hello,

Is there a way to double the number of turns before a city succumbs to a siege? Ex: If it normally takes 4 turns for a town with a wooden wall to fall to a siege is there a way to make it 8 turns for that same city to fall? Or is this a hardcoded limit?

I believe every aspect of seige time is hardcoded. Personally, I'd like to see them shorter.