Pyramid of Power

The Pyramid of Power is a huge golden pyramid in the Dark World in, which mirrors the location of Hyrule Castle in the Light World.

Features
After Link defeated Agahnim in Hyrule Castle, Agahnim sent him to the Dark World, and deposited him on top of the pyramid. Later, after Link defeated Agahnim in Ganon's Tower, Ganon fled to the Pyramid of Power. Link destroyed him, and rescued the Triforce, which Ganon kept inside the pyramid. A lower stage of the pyramid was also home to the cursed Fat Fairy, who gives Link the Silver Arrows and the Golden Sword. It may be that the top of the Pyramid was once the spot where the Triforce shined down upon the rest of the Sacred Realm, this being due to a picture of the Sacred Realm prior to its corruption from the manual of A Link to to the Past in addition to the steps that allow one to ascend to top of the pyramid from below and its location at the very heart of the Sacred Realm.

In the Game Boy Advance release of the game, the pyramid also contains the Palace of the Four Sword, a bonus dungeon for those who have completed multiplayer game.

A Link to the Past comic
The Pyramid of Power is briefly featured in the A Link to the Past comic. After failing to save Princess Zelda from Agahnim's clutches, Link kills Agahnim by deflecting his own magic. However, Agahnim, with his final breath, manages to transport Link into the Dark World, with Hyrule Castle collapsing as the Pyramid of Power is revealed.

Theory
Given that the Dark World is merely the corrupted Sacred Realm, it is theorized that the Pyramid of Power may, in fact, be the Temple of Light. The temple is said to exist in the center of the Sacred Realm, while the Pyramid sits in the very center of the Dark World, exactly where the temple is supposed to be. Both buildings are also shown to be the site linking the Light World and Sacred Realm, both in (for the Temple of Light) and in  (for the Pyramid of Power). Both structures also serve as the ultimate resting place of the Triforce. These many parallels increase the likelihood that the two structures are one and the same.