The first thought that came to mind about abusing it is pretty much what kinngrim said. Even if you have to visit all the flags as attacker to save it, you could make it ridiculously hard for an attacker to reach your flags (really long and narrow route) while you could let your defenders just hop down from some minor platform. Sure, ladders would be a counter for that though.
Also it's not really abusing the game mechanics necessarily per se, but unless a hammer of punishment is waved around like with custom banners most fortifications will be ugly as fuck. When you are fortifying an existing village I'm pretty sure that it'll often be a great idea to just wall of spaces between some buildings and use the existing buildings as walls. That said I bet people would just make giant penises too. Not really a problem, but just pointing it out.
Anyway, this is great. If it works well it'd be awesome if all the towns and castles would be redone so that towns have no walls and castles have just a minor somewhat defensible castle keep and no real walls at all. Players building the walls from scratch for those would be hilariously awesome.