I like it when I can set up a maze and use that juggling technique to keep changing the way that the enemies need to move to reach the exit.
Then, I set up one or more area bursts at the pivot point of the juggle so the enemies have to keep moving back and forth through an area burst. Also, it can be really good having an area burst as part of a mega tower as that seems to magnify the effect.
Bubble Tanks Tower Defense is a great game, and easily one of my favourites.
