Brilliant and very highly accomplished. It looks great, sounds good and plays very well. It fits very nicely into the 20 second constraint because the rounds are naturally brutal and short without feeling like they're being cut down to fit the time limit.
Maybe adding music for the start and end of each round and something playing in the background while you navigate menus would give it a touch more of a GB vibe? Not that it really feels like it's lacking.
Aside from that, the game is pretty near perfect. Great work.