Ancient Cities in Minecraft are located deep underground, in the Deep Dark biome, which is far below the depths of the surface. They are very rare to come by and will always be underground. The Ancient City generally spawns at around -52 Y level. Whenever you find a Deep Dark biome, its not always a sign that youre near an Ancient City, but its important to mention whenever you find one. A good way to help you find the Ancient City is by opening up the command page and typing in "/locate: AncientCity," and youll get nearby coordinates except for the Y level. As soon as you get the coordinates, start digging down until youve reached -52 levels, and try not to fall to your death. The only mob that currently spawns in the Ancient City is the Warden.