In case you don't frequent Major Nelson's blog, he announced yesterday that Defense Grid: The Awakening was the first July title for the Games with Gold program. The game is usually 800 Microsoft points.
I love TD games too, but I've never played one on a console. I'm not one to pass up free stuff, though. I just "bought" it myself, so I'm looking forward to trying it out later. These kinds of deals make me feel better about spending money on Xbox Live Gold to begin with.