5/5. You did A LOT of things right. I like how you can collide with enemies to kill them off the swing. and how the turrets stay around and you can see how many shots are left. This leads to interesting statergies, such as creating lines of turrets for defenense, or leaving the lane more open for collision kills. So much skill expression in just the first level!
The difficulty scale is pretty decent as is, though I would look into lowering it just a little bit. As other have stated, theres a lot you can do with this to make it a full fleged game, such as buying better turrets.
The base gameplay loop is so fun and original, that you have a lot of room for expansion. I really like the sounds as well, the music / sfx blend was nice where sfx didn't cancel out the music. The pews, are genuinlly fun sfx, surprised you found something that fits so well in such a short amount of time.