The Colosseum was primarily used as a quarry after the fall of the Roman Empire.
Once the amphitheatre stopped hosting regular spectacles in late antiquity, its stone and metal became valuable building materials. Earthquakes damaged the structure, while organized stone robbers removed blocks, marble, and iron clamps for reuse elsewhere in Rome.
The Colosseum was not used only as a quarry. Its spaces also housed workshops, dwellings, a cemetery, religious occupants, and, for a time, a fortified residence. That variety causes a common mix-up: “quarry” describes its most famous later role, not its original purpose.
Much of the material taken from the building went into churches, palaces, hospitals, and other Roman construction projects. The surviving structure therefore reflects both ancient engineering and centuries of later reuse.