Pretty good for a first game! The movement works well. Since you asked for constructive criticism, though, here are the two biggest problems I found:
1. The hitboxes/colliders are way too large. I could clearly see I didn't touch the enemies when I died, and I didn't touch the flag when I won the level. Usually in good platformers, the hitboxes are a bit smaller than the sprites, so there's no doubt the player hit the object.
2. Having to restart the game all over and redo easy levels just to get back to the harder level causes needless frustration. I'd rather just start at the beginning of each level when I die.
There's also a lot of little things that would improve the game, but those two are the most important in my opinion. Good luck with your future games! :)