There should be a few frames of invincibility after you get hit by one of the purple orbs the bosses shoot out. It seems unfair to be blasted back against the wall, only to immediately be hit by another purple orb.
Really good game, definetly one of the best on the site. Only problem I had was with the replay button. I don't need to know what level i'm on every single time I decide to restart a level >.<