Great game, excellent concept. Most tower defense games allow for tower improvements, a few allow for path construction, none other does so in such an elegant minimalist way, with no gold, mana, etc. The way towers must be improved by the same mechanics that outlines the path enemies must take guarantees quite a complex dynamical challenge. One must continuously update their towers by placing other towers on the map, which in turn might change the path enemies can take, which requires for different towers to be updated elsewhere, and so on.
I find it rather amazing how such a complex, dynamical, intricate gameplay emerges from such simple and elegant game mechanics.
However, this game could be even better.
1. Differences between tower and enemy types should be explained beforehand.
2. Towers could have more distinct roles. I find that the way it currently is, red, green and blue (not yellow) towers are pretty much interchangeable, that is, even though they attack in different ways, their effect on enemies is very similar. There are other ways they could attack that would make gameplay even more diverse: slowing enemies down; damaging over time; with area attacks; with lasers that go through enemies, etc.
3. Enemy waves cycle much to fast. If there were just five more seconds between them, there would be more time both to place more towers and to think about where to place them, building up to a more engaging challenge in the middle game. Right now it often feels like this game is more skill based than strategy based, when it feels like it should be otherwise.
4. An alternative would be to have different difficulty settings, even if the only change between them is how fast the game cycles between waves.
5. Also, I noticed that one gets the same amount of points when a tower is upgraded, regardless of how many towers were used in the process (3 to 9). Maybe it is deliberate, but I think it would be nice if those extra towers were accounted for.
6. Finally, I noticed one only gets 5 "Newground medal" points for the Green Mastery medal, whereas 10 points are awarded for red and blue ones. It doesn't make much sense to me, as they are all equally hard to get.
Anyway, this is still one of the best games I have ever played in Newgrounds. Congratulations!