Castellum

The cylindrical Mausoleum of Hadrian was built across the Tiber from the Campus Martius by the Emperor Hadrian as a mausoleum for himself and his family (and should not be confused with Hadrian’s Crypt which was, as I’ve pointed out, for a totally different Hadrian.)  A ramp winds up the cylinder……to the dark cork of the structure where the families were deposited.In 401 C.E., the complex was converted to a fortress and rechristened Castel Sant’Angelo.It is said that the Archangel Michael appeared above the castle, sheathing his sword, to proclaim end of the plague of 590 C.E.In the 14th century, complex was converted once again.  This time, into a papal castle (and occasional prison.)