The most tangible reminder of the ancient rectangular Castra Regina (Regen Fortress), where the name ‘Regensburg’ comes from, is the remaining Roman wall, which follows Unter den Schwibbögen and veers south onto Dr-Martin-Luther-Strasse. Dating from AD 179 the rough-hewn Porta Praetoria arch is the tallest Roman structure in Bavaria and formed part of the city's defences for centuries.