this is a really cool idea you have here! Playing with the idea of different dimensions is really cool!
The game's pacing is pretty good. I liked being able to buy upgrades and replay levels over again in order to purchase them. Until I upgraded to the graphics, it would have been nice to have the enemies/bullets be different colors, especially to distinguish between the different types of enemies. Maybe instead of the graphics being different colors, they could be actual sprite character (this could add to the story telling of the game as well!) Overall, good job!