Fun game, good music. If you kill a boss, but get killed by a creep, the all the text from the next level UI goes away, and you can't proceed any further. Happened to me on level 3.
Yes you have to destroy all the creeps to win, I will have to look into it a little more as well. I remember making this way back and I would like to make a new version in the future. Thank you for the feedback.