Players must defend military bases from waves of attacking aliens by positioning a range of tower-based weaponry. This game is dirt cheap (I think bbq got it on sale for 3 bucks). I have over 300 hours of play time on it so thats pennies per hour. Defense Grid is a huge time sink while you are waiting for the TF2 servers to fill up. You can play this game many times with the different modes that you can set up for each map. I hessitate to say it is a puzzle game because that could give you the wrong impression. There are many DLC's that can be purchesed on steam. There is a ranking system on steam so you can compare yourself to your friends and other top players in the world. PC Gamer "Defense Grid is such a charming and challenging experience that shouldn't be missed by tower defense fans. Even if you're not a fan, it may turn you into one." IGN stated, "It's an addictive time suck that will have you wishing for more once you've wiped up the last alien."
This Game is well worth the price even if it were tripled.