I'm glad to see a new Monster's Den, and most of the new features like stances are pretty great. But the graphic improvements weren't really necessary, and the interface suffers a bit because of that (harder to know just how much health/mana everyone has as a glance, harder to target a specific unit, harder to understand the equipment because of the very small icons).
I would suggest incrementing the cost of upgrading your starting money: make it cost 1000 at first, 2000 the second time, 3000 the third, etc. That way you'd run into diminishing returns and buying the other towers would be more valuable. It'd also up the difficulty, which is problematically low right now.
Also, most other tower defense games have enemies which are different in ways other than just hit point totals, and this one hurts because of that. Big differences in speed, movement type, powers, etc. would be good on enemies.
The core idea of this is great. I love the variety of characters and the ability to play as every enemy in the game. But the engine is really weak, and isn't likely to be fixable without a complete overhaul. That said, this could very well start a new line of games that improve on this idea, in the same way that there are tons of games that improve on the tower-defense or launch-into-space games. Creating a new genre is very impressive work; the fact that it isn't great the first time around is forgivable.
aside from the labeling problem the biggest thing is that this just doesn't have any real unique twist to it. It plays just like a very standard tower defense, without any mechanics that aren't in almost every other tower defense game. It needs unique touches.
It has some good ideas, but it needs a lot of polish. Fixing the memory leaks would be a good start, and making stages larger would also work. Try to make the number of places to put turrets larger.