Made me a fan of Tower Defense Games
One is eased into the game with easy early levels that quickly ramp up to harder situations and areas to defend. The graphics are good and the variety of aliens to kill and towers to built are varied and pose different challenges.
One great aspect of the game, if you buy it on Steam, is that you are linked to other friends who also play the game. You can see their high scores on a particular level and this creates an incentive to try and beat them. This further causes one to repeat levels, looking for different tactics for an overall strategy of gaining a higher score.
One of the best features in this game is the ability to go back to a previous checkpoint - an automatic save game point that is determined by the game. This allows one to undo mistakes, re-evaluate new tactics, and try different scenarios.
I'm totally hooked on this game.