Timeless River

The Cornerstone of Light (光のいしずえ Hikari no Ishizue?) is a sacred relic of Disney Castle that appears in Kingdom Hearts II. A large sphere made of crystal or glass that sits atop an ornate pedestal and contains swirling light, the Cornerstone protects Disney Castle from Darkness. It can be found in the Hall of the Cornerstone, a hidden room under Mickey Mouse's throne.
Although it is a powerful artifact and heavily protected, Maleficent takes advantage of a door that appears into Disney Castle-exile Pete's past after he is made miserable by her constant abuse. Using the door, Pete is sent back in time to when the castle had yet to be built, and the Cornerstone sat unprotected on a hill. There, he attempts to steal the Cornerstone, weakening the defenses of the present Disney Castle and allowing Darkness to penetrate the castle; the Heartless then invade and attempt to take the castle and its inhabitants prisoner. Maleficent's trademark thorns surround the Cornerstone and she appears in a vision to taunt Queen Minnie about her imminent victory.
Sora, Donald, and Goofy receive a distress call and arrive just in time to help Queen Minnie raise a defense against the invaders. After a quick trip to Hollow Bastion to seek Merlin's help, they discover the time loophole that has allowed Pete and Maleficent to weaken the Cornerstone's protection. By taking a trip through a new door that appears in the Hall of the Cornerstone that leads to Timeless River, Sora and his party manage to thwart Pete and Maleficent's plans and restore the Cornerstone of the present to full power. The Cornerstone also opens gates to Port Royal and Atlantica.

Cornerstone Hill
Waterway
Wharf
Pier
Building Site
Lilliput
Scene of the Fire
Mickey's House

Sora and his party enter the Timeless River on Cornerstone Hill, where they find both the Cornerstone of Light, unprotected but still safe and discover their new... er, old appearances. Signs around the area announce the impending construction of Disney Castle, and Clara Cluck, Clarabelle Cow, and Horace Horsecollar are around to chat with, as is a Moogle shop.
Heading to the right leads to the Pier, where the party meets up with young Pete for the first time and beats him into a pulp. Continuing on the other path will take the party to the Waterway, site of the battle with modern Pete for the Steamboat Willie, and the Wharf, where young Pete and Sora's group begin the final boss battle with Pete to finish Timeless River.
Also of note on Cornerstone Hill are four Windows to the Past that eventually open as the story progresses; each of these leads to a new area based on a classic Disney short film. From left to right, the windows are:
The Building Site, a construction site with a rickety platform and based on the 1933 short Building a Building.
Lilliput, a tiny town with a big cannon that can be activated by jumping on it. It is from the 1934 short Gulliver Mickey.
The Scene of the Fire, a big building with fierce flames that was inspired by the 1930 short The Fire Fighters and the 1935 short Mickey's Fire Brigade.
Mickey's House, which shows the inside of Mickey's House, decorated for Christmas, a la the 1931 short Mickey's Orphans.