Hi,
question, Why make a tower defense and not a Survival RTS like the upcoming Conan from petroglyph or they are billions?
I think the Ashes universe would be perfectly suited for a game like this more than a tower defense.
You could take control of a city or outpost of Centauri; expand it, build lots of defense, Harvest resource, Setup Ambush points, do some questing on the map to help you prepare for the invasion or something like that.
Any opinions?