The upgrade system is nice. It's got enough variety in the options available to allow for replay value and trying out different builds. Once I had explosions that were larger than the screen, and could still make them even bigger.
The game really is endless. With the right upgrades you could kill enemies faster than they can spawn, which unfortunately leads to having to wait for spawns. It's also possible to kill enemies before they even enter the box, causing them to drop their experience crystals outside, where they're unable to be picked up.
Overall, I think the game feels a bit unpolished and unfinished, but has great potential. I'd encourage you to keep working on it and add the more weapons you've always wanted to if you ever get the time. Also, perhaps make the player character face left when moving left?