From the Rubicon Development website:

“‘Mega Defender’ is a new twist on an old classic – Tower Defence. Sporting a swathe of creep and gun types, this all new game will keep you busy for hours. It started out partly as Paul Johnson’s hobby project and partly as a demo for our cross platform Raz0r engine, but we decided to finish it off properly and offer the game as a free download as a reward for coming to our site.

It requires a beefy PC with ideally a shader model 3 video card (SM2 is min spec). The game has been tested on XP and Vista. Unfortunately because it’s a free game we can’t offer tech support, but thus far we’ve not heard of any problems with it. Download size is a tad over 30 Meg. Enjoy!”

Website: rubicondev.com
Download: Here