You only need one button to play, either click or use spacebar. Tap to change direction, hold to stop and attack continuously. Backspace returns you to the menu if you want to bail out of a level.
The original proof of concept for this game was Chernobyl, a 48-hour Game Jam game I did back in college. While this is still nowhere near what I wanted to do with this concept, it does have a couple more of the features I wanted. I decided to stop development midway through because my eventual goal is to emphasize mobile couch multiplayer, which just doesn't work with Flash (you'd need to be able to register two taps at the same time.) Also, the long taps (for sustained fire) open up the right-click menu which is also not great.
Check out the Unity version: