You should probably fix the tileset, they have visible lines running down from the top. Also you should make a bigger playing field.
The bullet firing noise was annoying, you should probably change that to something much more pleasant to hear, try adding an animation to the gun and the skeleton's arrival.
Overall, the wave mechanic and the skeletons coming from the top was funny, if a bit suprising, and the pathfinding was a bit lackluster.