Triple Tower Defense First of all, I really enjoyed this game. I enjoy free-mazing, if that's even a real term, rather than having a forced path. When you combine that with the challenge of planning to create better towers, it becomes a truly strategic game.
Now, a lot of people are talking about how easy it is to juggle. I noticed, at least on the armorgames version, that there's a small price for removing a tower, as with discarding. That's effective for the first few levels, but not beyond that. So rather than trying to prevent people from juggling, I would recommend making the enemies get stronger faster. That way juggling becomes pretty much essential. Because really, that's what it boils down to in the end. There's a finite amount of time that a maze that doesn't juggle can last. At least make it so that a player can't last til level 100 easily juggling.
Basically, make the enemies grow stronger more quickly. Otherwise, great game.