This expansion contains four single-player campaigns from the original game "Cossacks: European Wars", and five – from "The Art of War": nine campaigns with 63 hardcore missions. Play additional 30+ hours of Cossacks! The campaigns allow becoming aware of historical events' large scale.
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