It plays like a Thunder Force game (with a Thunder Force-level soundtrack), but it's really a nod to all of the horizontal shooters of the '80s and '90s and more, containing references to Thunder Force 2-6, Gradius, Darius, Life Force, R-Type, Bio-Ship Paladin, Einhander, and even a couple of things I don't recognize but I'm sure are calling back to something.
You can download the game from here.
And just so you know what you're getting into, here's my one-life clear of the game.