Buy Age of Empires III: Definitive Edition - Knights of the Mediterranean

If you do not have a Steam account, you will need to download the client from the official website, install it and then create your account. Once you are logged into your account, click on "Add game" in the bottom left corner and then on "Activate product on Steam". After a few clicks on "Next", you can finally enter your CD key, the game will automatically be added to your library and the download will begin.

More information on Steam

You need a Microsoft account to download your game. Create one or log into yours if you already have one. Follow the steps on the Microsoft app to buy the game and it will be automatically added to your account. It will appear in "My Library" and you can access it anytime.