When the Moon shines bright in the sky over Amber at night, a great duplicate of Amber City and Castle Amber appears in the skies over Kolvir, floating upside down. Gravity is reversed within it, and it is accessed by a staircase that appears on those moonlight nights. Should the Moon be covered, it begins to fade, and if it vanishes completely, all within it fall, usually to their doom. The city is most solid during the three nights of the full moon, and most dangerous to access when the moon is only a sliver, becoming spongy and close to evaporating even when the moonlight is unblocked. Only on the nights of the new moon does it not appear at all.
Where Amber is What Is, and Rebma is What is Reflected, Tir-na'nog is What Might Be and Could Have Been. It is a place of dreams, visions, and illusions, where the wise seeker can learn the answers to mysteries, and the unwise seeker misinterprets what he sees and rushes off to his doom. Shadows of everyone in Amber, maybe everyone in all creation can be found here, and will interact with you, generally in ways unexpected. They play on your doubts, on your fears, on your hopes and dreams.
Three things usually bring seekers here. In the Hall of Paintings, every painting is a trump which can bear you to places you've only dreamed of...if you pick the right one. Deep 'below' the Dream Castle is the Pattern Chamber, which works just like the ones in Rebma and Amber. Well, except for the risk it'll evaporate under you and you'll die. And then, there is the Forbidden Gate.
The Forbidden Gate lies in a chamber which can only be accessed by climbing down a ladder from the center of the Dream Pattern. It is a thirteen-sided chamber with a huge rectangular door in its floor. There are four slots in the door which look like keyholes, although they'd require fairly huge keys. No one has ever successfully opened it. Once you climb down into the room, the way back up vanishes; hope you brought trumps with you or you can survive a long fall when the Castle vanishes. Runes on the door say in Thari:
Beyond this Gate lies power unimaginable, waiting for whosever finds the keys to unlock it.
There is more writing, but no one has ever admitted to successfully translating it. Fuyutsuki once claimed it was runes of the Sidar, but has never told the same story twice as to what a Sidar is.
During the Annadil Crisis, it was discovered that some Guardian waits beyond the gate for whoever might open it, a final challenge for them to overcome.
In recent years, the castle seems more solid and sometimes flickers into existence during the day. When this happens, pleasant, cooling breezes blow across the land, bringing relief from the heat. The cause of this is unknown.