This is easiest when done in local multiplayer. See "Count of Carcassonne" on how to set this up. You need to complete a building and score it by outnumbering another player in followers. In order to form an embattled city, you must create pieces of a city near each other. With your main player, make sure you make two pieces of the city and place followers in them. With your secondary player make one piece of the city and place one of their followers in it. Then use both of your players to merge the city into one and complete it. Once completed, you main character will get all of the points for the city and this achievement will unlock.