This is quite an interesting idea, being in a shooting range with from two direction. Feels like it's very convertible to a nice little arcade endless game with different upgrade and such. Speaking of upgrades I do need to give you credit for the upgrade system, it feels intuitive and fair.
I think the game do lacks a bit in the presentation department, that I don't really understand the game until almost the end of the first wave. Simplest solution would be adding bow and arrow animation that instantly conveys what was happening during the game.
Anyway, keep on the good work!