How can I restart on a world? What will be lost/kept?

In order to take a fresh start on an existing game world, go to "Settings" and then select "Restart". 

Note that all cities, units and resources will be lost with the restart. Inventory, coins of wisdom and war, heroes and 3 culture points (if you have 3 cultures points or more) will be kept. While battle points will be kept for the rankings, they can not be used for Victory Processions, they are only to maintain your position in the rankings. You may only restart if your beginner's protection has ended.