It's OK; but there are a lot of features that are usually expected but are missing. Such as, I would like a health bar above the main building so that I know how much time I have. It's hard to switch over the that display while trying to fend off a lot of enemies.
And the music reminds me of StarCraft. :P
@yogsloth: Yes, I was confused about that for a while too. I wish the game had explained it (or displayed it) better, because I realized that it was supposed to be in the center about 15 days afterwards.