Age of Empires: Castle Siege invites you to guide your empire through the Medieval Era on your Windows PC or touch screen device. Choose from one of several civilizations, including the Britons, Teutons, and Kievan Rus. Upgrade your keep, fortify your walls, and garrison defensive troops to protect yourself from marauders. Then, train an army to raid other cities, and command them in battle to dismantle your opponents’ defenses.

Age of Empires: Castle Siege was a free-to-play medieval massively multiplayer online tower defense game in the form of a Windows app, designed for Windows 8.1 and Windows Phone. Released in 2014, the app featured micro-transactions to aid in rapid development of a castle and to improve defensive and attacking capabilities. It was developed by Smoking Gun Interactive and Microsoft Studios, and was formally announced on August 26, 2014. Unlike any preceding Age of Empires game, Castle Siege takes the form of a tower defense game. Castle Siege was made available on iOS in 2015 and Android in 2017.

On November 13, 2018, Microsoft Studios announced that it would close the game on May 13, 2019, and since then, it is presently not working.