The Mausoleum of Theodoric stands on the northern outskirts of Ravenna, slightly removed from the other UNESCO monuments, and is worth the extra effort. Built around 520 AD for the Ostrogothic king who ruled Italy for three decades after the fall of the Western Roman Empire, it is architecturally unlike any other building of its era: a two-storey decagonal structure made entirely of large ashlar blocks of Istrian stone, with no mortar.
The building's defining feature — and the reason it appears in every architectural history of Late Antiquity — is the roof: a single limestone monolith of colossal scale, hollowed on the underside into a shallow dome. The quarrying, transport, and lifting of a stone this size in the early 6th century was an engineering achievement comparable to the construction of the Roman Pantheon. Standing beneath it and looking up, you are looking at a single piece of rock.
The interior originally housed Theodoric's porphyry sarcophagus; this was removed and the building repurposed as a church. The niches around the upper gallery, which may have once held statues, are now empty. The surrounding park has good views of the building's exterior geometry from a distance.