I sheet you not, asides from being the greatest representation/game of the First World War I have ever seen, it's an incredible TD game. None of this "glorified spreadsheet, press fast forwards half the time" Turrets have no statistics, act believably (compared to X-percentage-to-hit, I guess) and you can also control biplanes, tanks and the turrets themselves to get involved with the fight.
There's the rattle of machine guns, the every aching thud of artillery, and the hiss of gas as you fly above the battlefield in your biplane, strafing the enemy. It's actually reverantly horrific, though the fact the enemy explodes into a shower of clockwork saves the game from being totally inhumane. I really do think this is the greatest "vision" of the horrors of the First World War I've encountered, and it should be played for that reason alone. To watch a map turn from pleasant country side to blackened wasteland is pretty haunting.
If you haven't tried Toy Soldiers and have any interest in WW1 or tower defence games, try Toy Soldiers or Toy Soldiers Cold War.
(Also get it on X-Box 360, the PC version just doesn't capture the same feeling for a bunch of small reasons)
Orcs Must Die is also, of course, brilliant... but for some reason it didn't FULLY satisy me.
I really can't stand most "generic" Tower Defence games nowadays. I played Defense Grid as highly reccomended by a TD loving mate, but found it to be shite.
Also Revenge of the Titans is an extremely charming, funny and difficult TD game on Steam. Pixelly retro goodness that really comes off as authentic.