This is a fantastic game idea! I really enjoyed both the concept and the gameplay.
The only thing that disappointed me was that the game actually gets easier with each level. By the end, it barely offers any challenge. It almost feels like the levels were arranged in reverse order.
I would've loved to face a proper endgame threat while being stuck with just a single weapon in my inventory. That would've made for a much more satisfying finale.
Overall though, congratulations on both the idea and the game! Great job.