The game server is periodically reset to allow all players to have a fresh start. This used to be fairly arbitrary, but since the addition of Strongholds and Victory Points, each server reset has a planned duration at the end of which the winner is decided by the number of Victory Points.

The start and end dates of the world are announced at the top of Global Chat.

Screen shot 2013-01-27 at 4.04.11 PM
After a reset, the entire game is wiped and players start anew with level 1 cities.