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.

Plot
According to Prophetbot, the World is dying due to the loss of its sun, and Niko must return it to its place in the top of the Tower. Along their journey, Niko draws closer to the Tower, and its entrance room can finally be entered through the Refuge.

However, three pieces of the previous sun are required to progress further: the amber from Silver, the feather from Calamus and Alula, and the die from George. After obtaining each item, the Tower can be properly entered by standing on the X in the entrance room and closing the game.

After this, the World Machine attempts to prevent Niko from returning the sun, and traps them in a seemingly endless maze. With the help of The Author, they can escape the maze and find the Tower's summit, where The Player must choose to either shatter the sun and allow Niko to return home, or return the sun and save the World.

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.
 * According to a child in the Glen's grandfather, the Tower is constantly spinning in order to light up all of the World.