In the fourth level, I assumed you needed to send a minion to the upper-right and have it drop through the hole in that platform to make a platform on the spikes below. The minion didn't fit down there, it couldn't jump out, and it didn't die. I guess I could have restarted, but that's such a bizarre way to lock a puzzle.
Also, the music is obnoxiously loud and the boss should really have a dying sound effect.