Those holes all over the Colosseum are not battle damage. The outer wall was built from travertine blocks set without mortar and locked together with iron clamps, an estimated 300 tons of metal. After the Western Roman Empire fell, that iron was worth more as raw material, so people dug into the joints, pulled the clamps out and melted them down. Much of the stripping probably happened during the Gothic War in the 6th century, but the exact dates are not recorded. That is why the holes sit where the blocks meet.

• ✓en.wikipedia.org
• ✓artofmaking.ac.uk
• ✓the-colosseum.net

These holes are all over the Colosseum. They look like battle damage. They're not. Look closer. The holes sit where the stone blocks meet. The Romans built the outer wall without mortar. They stacked travertine blocks and locked them together with iron clamps, an estimated three hundred tons of iron. After the empire fell, that iron was worth more than the wall. So people chiseled into the joints, pried the clamps out, and melted them down. Then in 1349, an earthquake brought down the outer wall's south side. For centuries, the fallen stone was carted off to build churches, palaces and city walls. Rome's greatest arena wasn't shot at. It was stripped for scrap, one clamp at a time. That's why it's covered in these holes.