It's true that it offers an alternative exit, but that also means an alternative entrance (at least in this case it does). Having to guard both the upper and lower doors, in a mansion like that (with a plethora of windows that all have to be boarded up, etcetera), it would be a lot of work. The tower was ideal because they only had to monitor one entrance. Same goes for the colony, or any other guarded perimeter. Limit the entrances, so that you can focus your defensive resources there.