The gameplay is so fun. I chuckled every time that nuclear beam fell from the sky. Choice of art and sound effects suit the overall theme!
Maybe you could make discovering the enemies easier or add more of them per level. Also, how about when enemies see you, they try and go hide around one of the important buildings. Can lead to a fun chase and sometimes the player risking it all to kill the last remaining skeleton :)
Really impressive for a first game. Kudos! :D