Foundation medal for the new fortifications of Castel Sant'Angelo. Examples in gold and silver were placed into the foundation during the ceremony of laying the first stone, celebrated by Pius on May 8, 1561. Though unsigned, the medal has been attributed to Cesati on stylistic grounds.

Built on the Tiber River, Castel Sant'Angelo was originally a tomb commissioned by the emperor Hadrian and built between 135 and 139 AD. Over time, the popes took over use of it, turning it into a castle and fortress. Paul IV had first built pentagonal fortifications around Castel Sant'Angelo in an attempt to make it impregnable. However, that work had been carried out using compressed soil. On September 15, 1557, the Tiber flooded to a height of 18.9 meters (62 feet) above sea level and destroyed the earthen fortifications. Pius rebuilt the fortifications using the same plans, but this time using sturdier brick masonry. Work proceeded swiftly under architect Francesco Laparelli (perhaps due to frequent visits by the Pope) and was completed by 1565.