First of all: good game, kudos.
Great game in the beginning, lack of diversity on later maps.
Since there are more tower types available later on, this might sound somewhat paradox..., read on
On the first few maps, players can choose how to destroy alien waves in various ways.
My problem is, that maps do not only get harder (intentionally, thats ok), but they also involve less strategy later on.
The pressure the specific order of waves puts on players almost forces them to use a very map specific build (one or two mostly).
It feels more like cracking the map by memorizing it, and less like tactical adaptation or decisions.
While shielded enemies are a nice addition, they also contribute a lot to this problem.
I tried many combos, but it comes down to one dominating strategy (generalized, i know, but no less true):
Mostly Guns/Cannon, some Laser for the fast, some Temporals, just enough Anti Air.
Thats it, the rest often does not matter and is only decoration.
Ok there is exactly one map where concussion towers shine, but it seems this map was designed to support the tower and not vice versa.
I dont say its a bad tower, but often too expensive to get it running in a mace.
Some maps are doable with Temporals + Meteors, but exotic builds are the exception.
Not only are Guns/Cannons potent allrounders, in addition they often are the only working anti shield defense. And they are good against air too...
This combo makes them way too indispensable (and thereby boring).
Maybe you should change the way immunities work (boss shielded creeps should be affected by splash damage), make every tower more versatile or balance damage.
Dont get me wrong, the game is not imba damage/cost wise, but maybe some towers should be cheaper on upgrade level 1 and more expensive/poerful at level 3.
Progression order is imba.
Another way to solve this is a "programmable" tower feature. Vector TD(great flash game) has it.
Its not overly complicate, just setting the towers target prioriy to "weakest","fastest", "strongest","nearby","near map gate" etc... Of course there are presets (e.g. Laser->fastest), so casuall players dont have to worry about micro.
Another Solution might be various towers upgrade trees, instead o just making them stronger. (e.g. weak Multi-Laser, strong Single-Laser). So players can change the purpose of a tower midgame. Ant Buster (also Flash TD) is a great example.
Enough critic, keep it up.
Otherwise the game is superb quality, a lot of fun,
... and yes... i played any and all TD games on earth... some patches and yours will be TD King.