Hadrian's tomb first incorporated into the Aurelian Wall

Castel S Angelo 37, originally uploaded by mharrsch.

"As early as A.D. 403 the emperor Honorius may have incorporated the building in an outpost bastion of the Aurelian walls. In 537, when it was already a fortress, it was attacked by Vitiges and his Goths. In the 10th century it was transformed into a castle." - Rome Guide