My vote is for 100% health. That way it's obvious when the building will come up. I don't think you should be able to build an incomplete building, although I can't really put my finger on why. At any rate, something needs to be done about the current system. I was playing earlier today and basically had a fortress pop up out of nowhere. The aggressor was overpowering my construction team and defenses, and yet somehow I managed to build it anyway. That doesn't quite seem right.