When will a world close for good? What are the requirements?
If there are 300 or less active players on the world, a countdown of 28 days until closure will start.
However, on worlds with the world wonders active, there are additional requirements. The world will ony close when it is finished, meaning that a winner has been declared (4 Wonders built to and kept at level 10 from one alliance).
It will however not close right away after the fourth Wonder has been completed, but still run for a while longer. This gives the players the option to gain control over all seven World Wonders and thus win a final, powerful award for the world ("Master of the World"). This will then also be displayed in the player's profiles.
Once that is achieved, and there are 300 or less active players on the world, the countdown of 28 days will be triggered. You will be notified about the final closure time in the game itself.