Simple but cool idea, to have the player basically play several tower defense games at a time. It makes it more like a "campaign" than a single battle.
Maybe you could go in that direction, too. Some things that affect the global situation rather than a single planet, to tie those battlefield together some more? Maybe things like having the player "research" new constructions?
Maybe you could go in that direction, too. Some things that affect the global situation rather than a single planet, to tie those battlefield together some more? Maybe things like having the player "research" new constructions?