I like the charming retro aesthetic of the game, you use the limited palette well. Sometimes it was a bit hard to distinguish enemies from the background, if there were overlapping colors, usually with the cactus.
I suggest making the cactus slower when it approaches you from behind, so the player has a bit more time to react if they're in the middle of shooting.
Unfortunately I didn't get past the first level, it was like the game froze after opening the door?