Ancient City is a large structure — a sprawl of deepslate corridors, collapsed towers, and wool-draped platforms laid out around one central frame. That frame is reinforced deepslate, a block that exists nowhere else: mining it drops nothing at all, no recipe produces it, and pistons cannot budge it.
The frame is also the source of a long-running misunderstanding. What players call the ancient city portal is scenery. It has never activated, it leads nowhere, and no arrangement of blocks or items switches it on — its real function is to mark the chamber built beneath it.
The wool and carpet scattered through the ruins are the most practical thing in the building. A sculk sensor cannot hear footsteps on wool, so laying a carpet path ahead of yourself converts the whole ancient city layout into ground you can cross without waking anything up. Snowballs from the side chests do the opposite job on purpose: a thrown snowball makes a noise somewhere else, and a Warden that is already awake investigates the sound instead of you. Crouching the entire time is still the baseline.
What you carry out beyond the chest loot are the ancient city blocks themselves. Silk Touch lifts sculk catalysts, sensors, and shriekers away intact for redstone and decoration, and the Ward and Silence armor trim templates come from these ruins and nowhere else in the world. Cities never overlap one another, but two can generate close enough together in one large cave system that clearing the first leaves you within walking distance of the second.