The Romans established a garrison town they called Castra Regrina that became their power base on the upper Danube. Though the encampment covered an area of almost 25 hectares (62 acres), not much remains. The ancient Porta Praetoria, behind the cathedral, is the most impressive reminder, and through the grille beside the eastern tower you can see the original level of the Roman street, nearly 3m (10 ft.) below—which is why you often have to step down into the churches of Regensburg. Other Roman artifacts are showcased in this former monastery and include a stone tablet noting the establishment of the garrison, an altar to the god Mercury, and several Christian tombstones from the late Roman period. You’ll also encounter Albrecht Altdorfer (1480–1538), a Regensburg master of the so-called Danube School; look closely at his colorful canvases of biblical scenes and you’ll notice Bavarian landscapes and distinctly Germanic medieval towns in the backgrounds.