In the mid 1970s, the walls now visible in this cellar were excavated down to the bedrock. They show how buildings in this area developed from Roman times up to the late Middle Ages. Before construction began on the Roman fort, oak stakes were rammed into the ground in order to compact the prevalent mud and sand. Roman builders placed a dry-stone construction on top of these stakes. This wall survived for more than 1600 years. Sadly, Passau was struck by devastating floods in June 2013 and the whole construction was washed out away. The damage is visible below the yellow rope. Although conservators were able to use original Roman stone to rebuild the wall, they also had to use mortar to stabilize it. The Romans had used mortar in their day to cement the upper courses of the wall. The section of wall below the red rope is part of the original fort wall. Above the rope you can distinguish various construction phases from the Romanesque period to the late Middle Ages. Do you see the figure in front of the fortress wall? He is a Roman construction engineer, using what was known as a ‘groma’ to survey the landscape. A groma was a combination of a plumb line and horizontal crosspieces, used to survey right angles. With the help of ranging poles, the instrument could also be used to plot straight lines for the construction of forts, roads and towns. The colossal portrait you see here is of the Roman Emperor Probus, who reigned from AD 276 to 282. The fort of Boiotro was built during his rule.