A nice combination of bullet-hell style games and the 'select a level', 'collect coins to upgrade and get better' genre.
I always felt that bullet-hell games were a cheap challenge because it requires no effort on the designers part. It is essentially just throwing a bunch of objects on screen and gleefully watching as a player tries to dance through the chaotic madness. That's why, in most of these games, there is a dead zone that can be played with that a designer uses to test the game.
I figured out the dead zone in most of the levels making the game a little easier for me.
I actually did not upgrade anything until I had enough money for the floating coin collectors. Which wasnt until after the first boss.
There was definitely some thought put into some of the patterns of the bullets and how a player might avoid them expertly. So that was great.