Sky Keep

The Sky Keep is the final dungeon in SS undefined.

Entrance to the Keep
Link discovers Sky Keep after successfully passing the Silent Realm challenge of the Goddess in Skyloft, which in turn is enabled after learning the Song of the Hero entirely. Afterwards, Link receives the Stone of Trials, which he must insert into the empty hollow of a bird statue that contains an identical stone in one of its eye sockets. The statue then fires a cannon ball, which shatters parts of the land mass where the Statue of the Goddess sits, thus exposing the Sky Keep. It next shoots several orbs with Clawshot targets as their faces. Link can then use his Clawshots to enter the dungeon.

Themes and Navigation
The dungeon is located underneath the Statue of the Goddess in Skyloft. It has eight rooms in total, and to navigate them Link must alter the building's structure by using several terminals throughout the dungeon which operate much like a Sliding Puzzle, where he can basically complete the rooms in any order desired. Six rooms are designed in style and theme of each of the other six dungeons in the game, three of which contain access to pieces of the Triforce which Link must collect (the Triforce parts themselves are located within the Silent Realm). The other two rooms consist of the entrance and the room where one of the minibosses, Dreadfuse, is fought for a Small Key. Eight years after Ganon's Tower in The Wind Waker, this is the first final dungeon to revisit the concept of showing snippets of previous dungeons.

It's worth noting that some rooms have characteristics of more than one dungeon. For example, the Skyview Temple room has some drawings similar to those of the upper floor of the Ancient Cistern, the room where Dreadfuse is fought has many trees and plants, just like the first rooms of the Skyview Temple, as well as the interior of the Great Tree, and the Sandship room has some similarities with the Pirate Stronghold, like using a Timeshift Orb and walls appearing in the past.

After the Triforce is completely collected, Link wishes for the destruction of Demise and so the entire island containing both Sky Keep and the Statue of the Goddess breaks away from Skyloft and lands in the Sealed Grounds, where it is revealed to be part of the Sealed Temple. When it falls, the Sky Keep destroys The Imprisoned by crushing it. It should be noted that, because of this, Sky Keep remains inaccessible for the rest of the game.

Theory
In Sky Keep, there are three areas where the Silent Realm is accessed, each one housing a Triforce piece. Inside these areas, a parallel version of the Keep is displayed, with each one containing what appears to be an altar on which each piece sits. As the Silent Realm is believed by some to be the Sacred Realm, it is possible that this parallel temple could actually be the Temple of Light. This is supported by how, after returning to the Surface, Sky Keep sits in the Sealed Grounds; the Temple of Time seen in, which constitutes the only entrance to the Temple of Light, was built directly over the ruins of the Sealed Temple. Furthermore, the Sealed Grounds are in the center of the Surface, while the Temple of Light is in the center of the Sacred Realm. The two locations would thus coincide with one another.

Trivia

 * The Sky Keep is the only main dungeon in the series to not feature a main boss.
 * Though difficult, it's possible to skip the Earth Temple room completely through a clever combination of tile moves.
 * The audio theme of the upper area of Ancient Cistern is the only dungeon theme that isn't heard in Sky Keep. In the room corresponding to that dungeon, only the lower part's theme is heard.