Sschindylryn

Sschindylryn lies miles below the King’s Forest of Cormyr. This drow city is a significant trading power in the Underdark but a minor military one, at best.

Sschindylryn is a city that specializes mostly in divination and transmutation magics. They were very war-like at one point, using their portals to raid other underdark settlements and cities. A sound defeat by the army of Menzoberranzan curbed that practice.

Location
There doesn't appear to be any recent information about Sschindylryn, other than it's appearance on a map in FR:U 124. The map places the city in a location that doesn't match other sources, which place Sschindylryn under the King's Forest. The FR:U map places the city under the eastern farm lands. Assuming the location is wrong, the Plungepool discussion found at Volo's Guide to Cormyr mentioning Lake Daerbraun is the lake beside Sschindylryn.

Forgotten Realms Campaign Setting 3rd edition states that the city is located on the shore of Lake Thalmiir. Notes about the Plungepool describe how a river within the King's Forest falls into the underdark. The falls eventually lead to an underground lake known to the Drow as Daerbraun.

Description
From the layout in Annihilation (5th book in the War of the Spider Queen series) it says that Sschindylryn is contained in a pyramid shaped cavern having three sides each two miles long with the tip two miles high. In the outer edges of the cavern great trenches or moats have been dug around the city with several expanses of stone at various intervals around the city center crossing between the two areas. Slave quarters exist on the other side of these bridges away from the city itself.

The entire city scales up toward the city's center which is a giant ziggurat (a mountain of buildings and other structures). Many of the buildings are constructed from stone and brick (unusual for a drow city) and are fairly close together. They rest on stepped tiers, starting at the bottom of the cavern and climbing towards the ziggurat and continuing up. Sschindylryn's center (the ziggurat) is split into 9 levels and rises to a height of perhaps 2200 feet or a mile and a half.

Levels 1-3 are the Gate Houses. Accounting for approximately a third of the ziggurat's height, these levels house most of Sschindylryn's thousands of gates. And other than the city's many gates, there is no discernible way in or out of the great cavern.

Levels 4-6 are the Trade Levels which is the site of most, if not all, of the city's trade.

Levels 7-9 are home to the city's High Houses, or nobles.

At the pinnacle of the ziggurat sits the temple of Lolth.

The city is built on a kuo-toan ruin, both above and below the water, and even the new architecture retains a piscine style.

Features
Sschindylryn's Gate in Menzoberranzan (source - WotC web site): Sschindylryn's Gate is a portal to Oerth that was discovered by scouts from the city of Sschindylryn. Sschindylryn is a drow city under Cormyr known for its use of portals to explore and conquer other parts of the Underdark. During a battle with Menzoberranzan, the portal was damaged, and none from the western city know how to operate it or to where it leads.