Toy Soldiers is the archetype for what a good tower defense game should be.
As with all tower defense games, Toy Soldiers has you protecting an objective - in this case, your toy boy - from waves of enemies. Rather than having the enemies come from one path in front of your CPU controlled towers, they come in waves from your enemy's Toy Box. The option to control any of your towers at any time really allows you to immerse yourself into the gameplay and actually take an active role in the tower defense. My favorite, by far, was taking control of the sniper tower to pick off infantry from afar.
Another strength of Toy Soldiers is the vast arsenal and flexibility at your command. Towers are upgradable (as with most Tower defense games) but the upgrades really deliver something fresh to your arsenal. As well, flying around in a Red-Baron Bi-Plane is nothing short of great fun. The dog fighting is a little hard, but it doesn't detract from the overall package.
I have yet to play the multi-player component, but the campaign is nothing short of a great time with lots of replayability value.
I highly recommend this to fans of tower defense games. This will change how you look at the genre, and delivers a great game for a great price.