This game is a throwback to all the R-Type wannabe games from the 80's and 90's. The music had that perfect "Amiga 500" feel to it, and the game's style in general made me think of some of the old shooters from that era as well.
You get 4 different types of ships to choose from with different movement, fire, armor, and special weapon capabilities. Each one gives the levels its own particular flavor of challenge to it. Instead of being a straight up "Fly to the right infinitely while destroying bad guys who attack in predictable patterns" style of shooter, they offer you the chance to flip between left and right so you can turn around and shoot those ♥♥♥ holes that come at you from behind.
Much like R-Type there's a multitude of in-game powerups you can pick up which you find after destroying enemies or waves of enemies, and there's also a plain looking white box style powerup that apparently adds to your special weapon charge. (though I'm not sure how that works exactly because it seemed like I was able to fire my special pretty often).
Also if that's not enough nostalgia for you, there's a variety of in-game graphic filters so you can pretend you're playing it on a 4 bit greyscale screen, 8 bit greyscale, Vic-20, C-64, ZX Spectrum, Amiga HAM (Hold And Modify mode), EGA, CGA, and modern graphics (I may have left something out though).
It's challenging without being too frustrating, it's got just enough bullet hell without forcing you to become the zen master at dodging everything, and the graphics and music complete the experience.