What, just 2 more levels? Then I'll be finished after a week and won't know what to do with the resources
How about 20 or 50 more levels? Each wall should cost 30 million (maxed auto age storage) and the discounts from parliament and research should not apply or the prices should be correspondingly higher so that you can only build the wall with maximum research.
To prevent the walls from becoming infinitely strong, this could be implemented as a kind of prestige level. Each improvement coats a small part of the wall in gold. As gold is a very soft metal, only 10 Hitpoints are added. This would give us a place to destroy resources for a while without disturbing the off-def balance.