Nice work! Here are my suggestions and what I noticed:
- Sometimes enemies get stuck on walls and do a little vibrating bounce.
- I encountered one level 3 times in a row, perhaps make sure, for example, if lastLevel == nextLevel then increment nextLevel?
- No sound. I don't mind for a start, but even a nice little "pew pew" sound would be nice :)
Otherwise, good start! I like the enemy variations and arcade style, as well as the occasional breaks from combat. :)