Cave Buster is a short retro-style platformer which perhaps could be classed as a Metroidvania if it was a bit longer. You get a few upgrades that let you reach other areas, but there aren’t many areas (or many upgrades) so it doesn’t feel like you get to use them enough to consider it as that sort of game.
It is fun while it lasts, however. I’m assuming it was the result of a Game Jam or something, with the possibility of using the engine to create a larger game? Certainly, all it needs is more of the same and it’d be well worth a purchase. As it is, for free, it’s nice enough for a quick run through, it just ends (with the collection of a burger for… reasons?) leaving you wanting more.