Porta Praetoria
Built in AD 179 under the Emperor Marcus Aurelius, this is one of only two preserved Roman gates north of the Alps (the other is the Porta Nigra in Trier). A rough-hewn limestone gate, it originally served as the northern wall of the Roman legionary camp Castra Regina. Today, the most visible remaining sections are the exposed archway and the neighboring tower on Unter den Schwibbögen street, but if you look through the grille on the gate's east side, you'll also see a section of the original Roman road, about 10 feet below today's street level.
