It starts like your normal tower defense game. You build mazes using several upgradable towers to try to prevent the enemies from moving from one side of the room to the other. That's where normal ends with this game.
This game has all the addictive elements one could look for in a game. As you play and either win games or lose games your score at the end of the game is given to you as credit for purchases to improve your towers the next time you play around. That gives this tower defense a replay value unlike most others as it adds a reason to play again. You can't complete the hardest setting the first time you play. In classic leveling fashion you have to build up your upgrades in order to be powerful enough to beat the later levels.
The game also has a list of achievements attainable that it will grant upon completion ranging from grind out achievements like beat the basic level 10 times to more difficult achievements like beat a level without upgrading any of your towers or without using any slowing towers or without losing a single life. All of these achievements add fun new goals to try and reach which provide hours of fun.
Also they are constantly adding new achievements and level. They recently added a hidden boss that sometimes spawns while you are playing and an associated achievement if you can defeat him.
They also have a random level generator that creates semi-random obstacles throughout the map to give countless new ways to have to path around the map. It's available in a free trial demo but well worth the 3 or 4 bucks I paid for it.
Homepage : Robo Defense