BigTree Defense is a tower defense game with a somewhat unusual structure.
It's quite simple, really: You grow a tree (or perhaps are a tree), and the turrets are "flowers" growing from its branches.
This is a pretty fun idea, and I've always liked it. (I'd meant to review this game years ago, but never got around to it.)
On the other hand, while your tree can increase in size and become pretty impressive, I find the game's balance a bit iffy.
Specifically, each time I've played the game I've stalled out a couple levels in, because the enemies get dramatically stronger around the third stage or so.
I suspect part of the problem is the nature of the game's enemies and waves. You see, the enemies are insects that are apparently invading through holes in the sky or something, and when they successfully deal damage to you and then escape, they retreat and spawn new enemies. It makes for a very tricky gameplay experience (one enemy getting away can mean lots more enemies later), and I think the enemies could have used a bit more balancing.
Also, while I like the concept, the art of the game is... a bit underwhelming. The turrets and enemies are fine, but the shape of the tree and its branches is... ugly.
If you're better at this sort of game than I am (not unlikely), it might be more fun than it generally is for me. As it is, it's fun to play with until things get unreasonable a few levels in.