When you register and have an account with a user name, they automatically start you on a world. After that, you can continue playing on that world, switch to another world (as Lucifer outlined) and start a new city, start cities on all the worlds of the U.S. servers (currently 22), go back and just play your original world or play all or any number of the worlds you've started. The Diamond inventory you have (which you can augment by winning some in the game or buying more with real money) is shared by all the cities you have, although you can spend them all in one place if you want. All other inventory (coins, supplies, goods, medals, Forge Points, buildings, etc.) is fixed in the city where it is built/earned. There is no contact between the cities you have on the various worlds, unless you happen to end up in guilds on 2 different worlds that are paired against each other in a weekly Guild Expedition championship.