Age of Empires: Castle Siege is a medieval MMO tower defense game in the form of a Windows app, designed for Windows 8.1 and Windows Phone. 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. Source: Wikipedia, "Age of Empires: Castle Siege", available under the CC-BY-SA License.