In this basic tower defense game, you'll have to try and prevent enemy minions from reaching the path's end - otherwise you'll lose lives and (obviously) also the game if you don't have any lives left.
Like all tower defense games, the key to surviving the waves of enemies is to place the right kind of tower in the right kind of area. However you'll also have to manage these towers after you placed them, either through upgrades or by selling them in the right moment (so you can purchase even better towers). Once your legion of towers has defeated an enemy you will receive a specified amount of money depending on the type and strength of the defeated enemy. You will receive an additional cash bonus after successfully surviving each wave.
See below for further details on each enemy type that you might encounter.
Standard tank
The "beginner tank". You shouldn't have much trouble defeating those...
Enhanced tank
Slightly faster and more life than the standard tank. They start appearing from the 2nd round on.
Armoured tank
This fortress appears during the 4th wave and is rather slow, but it can withstand a remarkable amount of damage.
Speedy tank
If you make it to the 6th round, you'll see these annoying minions. They don't have much life, but they're hard to hit!