Virtually all of the assets used by this game seem familiar.
Maybe a little too familiar.
Heard of a game called Cave Story, by chance? One of the great indie classics? Originally freeware, but got a few commercial releases later on down the line?
Don't get me wrong, the game's not too terrible, but... well, let's be honest here, you could make your own custom sprites and music and it would still be a decent game. Right?