I'm surprised no one's just gone and tore somebody elses house down yet... oh wait.
I like the center square idea, with an MCB castle in the center for just doing whatever, and we can build our places elsewhere. Then people can choose if they want to go build a fortress someplace or just put a place near the city walls. We can cooperate to make it out of obsidian or something, bedrock just seems like cheating. And remember, only the outer layer up to like 3 high has to be obsidian, anything higher is out of blast range and everything behind it is protected. It's not that much Obsidian to jsut armor a regular wall like that.