It's a good concept and the execution is not too bad  or too great. I liked your choice of audio as well. I also liked that I could stand on either side of a platform, although it took me a while to figure that out (due to the colour blending in with background)

A few suggestions that come to mind are-
1. Create a learning curve that is less steep- the level design could gradually increase in complexity rather than having challenging bits from the very beginning.

2. The background colour and the player bullets have very similar colours- it was hard to see where my bullets were going. You may want to make the bullets better visible like the ones enemies are shooting- those are bold, contrasting in colour, and thus very clear to see.

3. I think you forgot to mention in the controls that Space can be used for jump. I pressed it out of instinct and it worked!

Overall, it is has good potential; polishing it further can make it more enjoyable.
Congratulations on your first game jam, and nice work!

Nice game! It's brutally hard though so if you're looking to expand in the future a good feature to add might be checkpoints. Good job though!