Cobble
As mentioned in Phantom Hourglass, the Cobble Kingdom was once inhabited by the ancient Cobble race. The remains of the Cobble Kingdom are found hidden beneath the water on the Isle of Ruins, and there is found the seventh dungeon, Mutoh's Temple.
Basis for New Hyrule
Some believers in a New Hyrule after the Great Flood site point Cobble Kingdom as the link between the old kingdom and the new. This is because several elements that are present in both Ocarina of Time and several of the 2D games that potentially have a post-flood placement are also present in the Cobble Kingdom.
The Isle of Ruins contains the graves of Six Sages, showing a possible connection to the Six Sages (excluding Zelda, the Seventh Sage) of Ocarina of Time. Such a connection could explain the preservation of the names of the Sages in Adventure of Link, where several towns are named after Ocarina of Time Sages.
The Cobble Kingdom's Royal Family was guarded by four knights. This story element is present in Four Swords Adventures, where four knights guard the Royal Family of Hyrule.
Both the Cobble Kingdom and the Hyrule of Adventure of Link contain a Maze Island.