Ancient City

An ancient city is a palatial structure found in deep dark biomes at layer -51, harboring chests containing some items that cannot be found anywhere else.

Environment aesthetics
An ancient city features a very large palace that stretches throughout a deep dark biome. The palace is made up of long corridors with some rugs made of blue, light blue and cyan wool or carpet floors to prevent vibrations as well as some smaller ruins off to the side of the main corridors, which contain between one or two loot chests. The city has statues and the center features a frame resembling a warden's head, where there are reinforced deepslate blocks, which is an unobtainable material in Survival mode. Other unique blocks such as soul lanterns and candles as well as different forms of deepslate and sculk can be found here.

Some structures resemble elements of the game. The city center bears a resemblance to early iterations of the warden's design, and other structures resemble pillager outposts and woodland mansion rooms.

Overall view
The following three renders are the same instance of ancient city.

Redstone circuits
Underneath the frame at the city center lies a series of hidden basements with naturally generating redstone circuitry.

The secret entrance is located at the base of the structure, underneath the wooden bridge between the city center and the wall. There's a piston door that is controlled by a sculk sensor in the redstone circuits in the basement.

File structure
All ancient city structures found below are located in the folder client.jar/data/minecraft/structures/ancient_city, data/structures/ancient_city.

Loot
The loot in the chests includes several useful items for exploring the ancient city, such as snowballs and the Swift Sneak enchantment, as well as items that can't be found anywhere else, such as echo shards.

Trivia

 * The ancient cities were inspired by Mayan and Roman ruins. In some concept art, Narrator revealed that the loot in ancient cities was originally going to be stored within jar-shaped pots, screaming vases, and stone chests.
 * The intention of the ancient cities was to capture a feeling of something that feels like it was once bustling with life, but is now vacant and lifeless.