Good game but a bit annoying at times. It seems I was lucky to get the right upgrades at the right moment. Otherwise you could be forced to restart the entire game as you lose all of the round's money if you lose (so you end in an unbeatable loop).
Also the ring/orb thing is a bit weird. The enemies will continue to go forward even if the ring/orb is behind them. Sometimes it just teleports back up which can be a game saver.