[licker]

The supply capacity of the wall is not the provincial supply, its the amount of supply that exists when you are beseiged. However, from my recolection, the wall's defense was so terrible that you wouldn't Last long in a seige anyway...

As to tearing it down, select a comander and the 'construct building' order, on that there should a 'demolish fort' option, along with 'construct [foo]' if they are magey or priesty, and 'demolish [foo]' if you have a temple or lab.