Really great concept and well executed! Nice arcade feel to it too... I see lots of elements could be added to this.
The gameplay is really well balanced and it all feels solid - great job!
A few detail points I'd suggest:
- When the enemies appear, they suddenly stop dead. It'll be best if they eased to a stop.
- Auto-fire might be nice as the fingers were getting a bit tired, especially after repeat play
- Be nice to have pick ups, like shields and power ups etc...
Overall though, superb concept and really well done - a really playable game themed for the Jam. Great job!