Maetri is called the 'Forever City', with good cause. The maze of streets and homes, shops and people never truly ends; it only blurs slowly into the estates of the nobles at the fringes. Sometimes the maze does not end at all, leading out into Shadow cities even as Arden runs through Shadow forests. It is a distinctly classist place, with infinite shadings of cultural value. The lowest class is that of the beggars on the street, who may not lay a hand on a noble without most likely losing it. The highest class is the high nobility. In between are the peasants who tend and work the nobles' estates or eke out a living in the city; the shopkeepers and traders, some barely surviving, others fat and rich; the soldiers, who keep order in the streets; and the artists who, under the patronage of the nobles, are slowly bringing about a cultural renaissance.
One day a year, the classist system does not exist. The Mardi Gras of Maetri is a time when everyone takes to the streets, in costumes or in masks, and in the anonymity of their disguise enjoy drinks provided by the vinyards of the nobles, dance around and enjoy themselves, and eventually are led by the most charismatic of the partiers ('The Kings of the Mardi Gras') to the Pattern, where it sprawls sunk perhaps three feet into the stone, glowing silver and sparked with red. A few people always try and walk it and burn to nothingness. The party around the Pattern continues for the rest of the night, producing a number of erotic encounters.
That's Maetri. But there is another side to things; Clarenne, a dark and twisted city on the underside of the world. Since Maetri is flat, this is the literal underside.
Clarenne can be approached directly through Shadow, or one can slip through the sewers from Maetri directly to the other side. Usually, the access to Maetri involves a sudden dropoff in the sewer bottom; when you step out into it, you walk around to the underside.
Clarenne is occupied by horrors, though some of them take the form of humans, or human children. Others lurk in the Corners or drift through the skies above. The world is an empty parody of Maetri, with the buildings ruined and overgrown, and a pale sun floating above behind the soft gray clouds. In the darkest tumbled ruins are the Universities, little-understood gatherings of creatures to which belongs the nominal rule. In the center of the realm, there is the other side of Maetri's Pattern; you can see Maetri's sky through the gaps between the lines.
Three noted personages of Maetri are the Mayor and two purported Amberites: the first his adopted son, Phillipe (found in the center of Corwin's Pattern), and the second Camille (who was not.)
There is more information available on Patterns.
Shavings of True Stone