Defendless

You should be able to combine 3 rocks to form a high-tech "Stack of rocks". Which would block flyers.

Placing a tower over another one should make them swap.

A decent, but there's a few things that would make it a great one:
* A save button, for those times when you have a great maze going on and need to leave
* The ability to switch between setups, so you don't have to rework everything (and then laboriously put it back together) for a flying wave
* Some differentiation between creatures beyond the graphics
* A modicum of story

Reminds me of the good old Gem TD in WC3.
You should balance flying a little more, hard to get far even with spells. Or perhaps make it so you can actually chose from towers, unlucky towers (lack of amathyst) will make it impossible to deal with them.

I swear flying waves are the boss waves. Every time I get to a flying wave I lose at least 5 times before my upgrades give me enough of a boost to beat them. The sheer HP they have combined with the inability to block them makes them at least equivalent of boss waves in other TD games.