I liked it a lot!
I love shooting up games and this has a strong similarity to Defend Your Castle also, making this a remarkable game. Really great game! I would suggest a save option though. I need to hit the sack and I don't want to lose my position haha. See if you can get in touch with the guy who created Defend Your Castle to find out whether he'll share his secret or not... it's very helpful! Great job! Keep it up!