So this game has one tower, the laser tower, that is simply way overpowered. To beat this game, all you really have to do is to last until you can get a laser tower. This is not a hard task. This strategy will work for every difficulty setting except the hardest one. Even on the hardest one though, the laser towers are the key to everything.

Basically you build 2 or 3 pistol towers on the straight vertical section of the map, and upgrade them evenly. Keep track of the selling values of the pistol towers and when you have enough money by selling the towers, sell them and buy a laser tower. Just keep upgrading that laser tower as money comes in and build a freezing tower so your laser tower can hit zombies more easily. This strategy should carry through pretty much the entire game. Build 5-6 of these laser towers and fully upgrade them and then start saving for missile towers. At this point, the game is basically won so you don’t have to worry.

On the highest level of difficulty, you have to do some fancy selling/rebuying to not leak and win. It seems like the hardest difficulty is endless but once you get settled down with 5 laser towers and 1 or 2 freeze towers, you can just save for missile towers and kill everything. Timing is key on the hardest difficulty.

Good luck.