Ancient City

An ancient city is a palatial structure found in the deep dark, harboring chests containing items that cannot be found anywhere else or that help a player avoid a warden. It always spawns at y level -52.

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 gray wool 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 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 can be found here.

Redstone circuits
Underneath the frame at the city center lies a series of hidden rooms with naturally generating redstone circuitry, such as piston doors and logic gates. The secret entrance is located at the base of the structure, in front of the frame. There is a sculk sensor above it. Each variant of the city center uses a different redstone circuit to conceal the entrance. Sometimes there is nothing in front of the frame. Other times, there can be a chest or a a set of walls in front of the frame or even sometimes

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

Blocks
An ancient city is the only place in the Overworld where soul sand, soul soil and polished basalt can generate naturally. It is also the only place in the game where a skeleton skull can generate naturally.

Loot
The loot in the chests includes several useful items for exploring the ancient city, such as snowballs and the Swift Sneak enchantment.

Trivia

 * Some statues scattered throughout the ancient cities bear a resemblance to early iterations of the warden's design.
 * There are structures that resemble pillager outposts and woodland mansion rooms.