Epic game! I was happy to come across a pixel game maker entry. Polish was immaculate, art is incredible and gameplay is top tier. The bosses were awesome, contra-esque, right up my alley. Only addition I would make to the gameplay is upgrades, obviously time constraints made that tough, but for a future version of this game. All around awesome project, just missing theme/story stuff. I'm not sure if I accidently skipped any story in the beginning. I personally don't mind jumping right into the game, but for jam purposes I would have pushed the main character's story a bit more.

Far and away the best part about the game is the atmosphere. Immaculate tone setter for the adventure, and every element of the presentation from the music to the UI engages you. 

When tackling such a well worn genre as you've chosen, its important to understand how to successfully execute on the subtlety's of the genre, which is a very long very kind way of saying you should have the option to continually shoot if the shoot button is held down, ESPECIALLY with enemies and obstacles that take more than one shot to destroy. Similarly I would say the most glaring determent to player retention is a lack of noticeable progress. The environments changing mid level is gorgeous but it seems to happen at random as just another thing that happens. Something simple like a countdown timer showing how long the player has to survive for or a progress bar would go a very long way for player engagement. 

The core game and how it is presented is absolutely solid and you should be proud of the work you've done, but small details are oftentimes just as important as the big picture.