Killing Floor is a Co-op Survival Horror FPS set in the devastated cities and countryside of England after a series of cloning experiments for the military goes horribly wrong. You and your friends are members of the military dropped into these locations with a simple mission: Survive long enough to cleanse the area of the failed experiments!
The CD Key / Serial is sent as a scan of the original retail box. This proves your 100% ownership of the product in any case. The scan is sent by email as well as kept in your orders archive so you can return to it anytime in the future.
Play on Steam: The key provided by http://www.topzonekeyonline.com is an official Steam Key (www.steampowered.com). It is activated via Steam, enables you to play on official Steam servers and your game will be kept updated with the latest patches via the Steam platform.
To activate our key via Steam simply download Steam, create a new account or login into your existing account, click Games and Activate game on Steam.
For 2 cents extra, I'd rather the peace of mind of going through Steam, thanks.
Not exactly sure how this is a deal.