Great game, but the first thing I noticed is the difficulty curve is awful, most of the time I have to retreat 3 times before killing the next staged enemy. Other than that, it's a super fun game.
You need some of everything to get a higher population. The next version does a better job of explaining everything, but it wont be out for a little while. Sorry for the confusion.
@koaaltje You have to put each one carefully (zoom in if you need to) in the corners, then you will be able to put 1 in the middle, making 5. (Using H is a good idea to complete it)
:O