You have to get the following:
Obtain a 'Second City Deed' by completing the necessary quest or purchasing it in the shop. To be at least level 7 (Baronet). To conquer and control a plain. To reinforce it with 250 supply troops and 10,000 each of gold, wood, stone, ore, and food. To have at least 10 friends on Facebook playing Kingdoms of Camelot.