To earn this one, you need 7 cities within 6 hex of where you build Jebel Barkal, the achievement will unlock at the start of the next turn after you build the last city. Now, when counting, dont count the hex the wonder is in. Its Six Hex outwards. You need a desert hill tile to build this, I used a map with lot of desert so I could have my capital nearby, build Petra really quick and then this one. Remember that you cannot build cities within 3 hexes of each other and plan accordingly.