Cossacks: Back to War contains all of the gameplay features of Cossacks: European Wars and Cossacks: The Art of War, this is a complete care package for all current and future Cossacks fans. This add-on contains two new nations: Switzerland and Hungary.
Open the Steam client, login and locate the Games tab on the top menu. Click this and then select “Activate a Product on Steam…”
Enter your unique code that's displayed on the 'Orders' page of CDKeys.com and follow the prompts to activate.
Your game is now viewable in the “Library” tab and is available to download/install when you’re ready. You can find additional support for the Steam client here: https://help.steampowered.com/en/wizard/HelpWithSteam