Great idea, well executed, bravo! One of the few strategy games I've come on and been able to just enjoy. The controls are simple and it's easy to get the hang of (for me anyway.)
Tip:
Pick up minions, drop them off somewhere they'll win fights by themselves with. They'll become a little horde you can use. If you do this a couple times usually the entrances will be covered well. If need be, grab the nearby horde and drop them on the ship to defend. You need superior numbers to take down the enemies efficiently, so keep that in mind.
@The author
The first time I played I almost quit as I believe I ran into an invincible version of the black monster. The second time I played the level they died just fine/easy. Unsure what happened but i did have one who ate an entire mob. I had literally assumed they were supposed to be unkillable after that and I was supposed to finish the level before they came (impossible.) Luckily they started dying. Maybe I was just bad, who knows..