This is a very interesting version of TD.
The 1st level: easy
The 2nd level: increases in speed and difficulty
The 3rd level: about the same
The 4th level: easy
I have no complaint about the difficulty of the missions. I will say that it isn't a traditional version of a turret/tower defence game. In most TD games, the player must stretch his/her defends along the path the enemies must take (atleast I do). In addition, the enemies fight back against the turrets adding to the challange of creating and maintaining a defence.
In order to beat the 2nd and 3rd mission, the strategy must change. Instead of separating the turrets along the path the enemies must take, all turrets must be concentrated together on either side of the path. The purpose: concentrate the turret fire to eliminate enemies faster. The less time it takes to destroy enemies, the less damage they can deal to the player's turret, and the more time it takes for the enemies to destroy the player's turrets. This means that the enemy can not destroy as many turrets as when your turrets are separated. Hence the expression, united we stand, divide we fall.
Another thing: keep your turrets away from the entrance by about 5 unit spaces. This will maximize your turrets range.
Finally, you should consider using the 2nd row of spaces from the path. From this point, your turrets are still well within range of enemies on the path and will maximize the amount of firepower despensed at one time, eliminating the enemies much more quickly.
As for the last mission: aim rockets carefully at the enemy PAD's, and stay at range from enemies.
Simply put, concentrate your turrets at one point -- on either side of the path, fear not the 2nd row, build power plants often, upgrade ye 2nd line of turrets (if you have to), and aim like a sniper.
Critique: I believe it requires a toturial. Once you know how to play the game, it is fairly simple to beat. I like the advanced strategy behind this TD game; vital placement of turrets on the map makes for a good challenge. Music is okay. Graphic are low but fair. Good work!
Nice game.