Tower

The Tower is a structure within the World that houses the Sun. It is located at the center of the World, surrounded by the Refuge. It is tall enough that it can be clearly seen even from a long distance away.

Details
According to Prophetbot, the World is dying due to the loss of its sun. Niko can try to save the World by placing the Sun at the top of the Tower. The Entity warned the Player though that their choices will affect Niko and they only have one shot.

Trivia

 * According to Silver, the Tower has existed "from the beginning of time." Whether this refers to the Tower in the Old World or simply the World is ambiguous, though it could very well be both.
 * The Tower is a very cryptic structure, with seemingly impossible room structures and loops that require outside help to overcome. However, this may be due to the World Machine's influence.
 * The Tower's power is strongest at the summit, where the Sun is placed.