Just different enough to catch the eye, the oaken door, bound in silver and gold, sits tucked away between two other doors of no import. Of course, there had never been a door like that there before...
Yet the one who walks in Darkness shall guide the Hand of Light to open the final door, where Watcher becomes Warder as Light grows weak and Time runs to its end when the cycle falls to darkness and despair, where the Champion betrays the Martyr, and ill omens of the night run rampant in the Eye which watches.
The Hall of Mirrors is indeed the final door, the doorway to the past, present and future, the dorway to the end and the beginning. It is the way to the final battle, the pathway to salvation or annihilation.
Created when the Primal Pattern was drawn, it is a physical representation of all the knowledge and memories Dworkin has. As Dworkin held the Mind's Eye, and drew the Unicorn's Pattern, for that time he held all the knowledge of the universe, past present and future, in his mind. The Hall of Mirrors was his mind's way, using the magic of the Eye and the Pattern, of compartmentalizing all that information, so he wouldn't go entirely insane. Indeed, those who walk the Hall walk through Dworkin' mind.
Because the Hall of Mirrors came into existence before the Primal Pattern was broken, it was not affected by the event. And so besides the Heart's Eye (which is now tainted with the Serpent's madness), it is the only artifact that still holds its full power. Indeed, it is the one artifact that can lead the charatcers to the Serpent, when the times comes.
Dworkin's addlepated state, assumed insanity by the family, is indeed not insanity at all, but a function of having had his mind literally split apart into thousands of tiny pieces. He can remember large quantities of information at once, but it isn't always related. As a result, he seems perpetually confused, and quite schizophrenic. Dworkin has no idea that, for all intents and purposed, he is the Hall of Mirrors. He believes it to be a function of the Tir.