Little tip for the final mission, as cruel as it sounds let the darkness consume your foes and focus on the Gates and the shipyard, else you'll spend more time on fending off the darkness, all four allow you to capture and fend off as you need.
Stuck in 15 at the time I'm typing this and all I've got to say is, I LOVE THIS GAME! It's mind twisting at some points and so easy the answer literally gives itself to you at others, and the difficulty doesn't just scale up it goes up and down as you progress, I find that refreshing. 10/10 in my opinion!