(aging or fading now, but a good five year run anyway -- and still lots of good resources there in the archives)
Kongregate! Forums (and other pages on site). http://www.kongregate.com/forums/90-kongregate-apis
Storm Alligator. The blog hasn't been updated in a while, and hopefully that means their working on new versions of Demons vs. Fairyland! http://stormalligator.com/blog/
http://casualcollective.com/ Games and game talk aplenty.
Kotaku is great, and not bad on Tower Defense.
Game Jolt -- great showcase for indie games. Some TD in http://gamejolt.com/games/strategy-sim/
Not a lot of action in Google Groups on defense games, but if you're diligent you can pull up some good threads.