Minecraft wiki entry for Ancient_City, associated text: 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 set of walls in front of the frame.