lo-res version
~~~~~~~~~~~~~~
+ walk behind some sprites (columns on level1)
+ enemy movement
+ animate enemies?
+ check raygun count before shooting
+ fix background tile when get item
+ fix keybindings/help menu
+ better start/stop level music
+ implement leaving level
+ animate death
+ game over screen if out of lives

hi-res version
~~~~~~~~~~~~~~
+

mars
~~~~
+ convert to full tilemap
+ mark completed levels

title
~~~~~
+ actual menu?