After the climax of its power, internal struggle weakened the military position of the Roman Empire. A series of attacks in the 2nd and 3rd century AD forced an adjustment of the military strategy in central Europe. Instead of further expansion, the borders of the empire were increasingly fortified. In Germany this led to the construction of an impressive naval fleet on the rivers Rhine and Danube. Several of the boats have been excavated and our team has attempted a detailed reconstruction of two types of vessel, the "navis lusoria" and the "Oberstimm", down to the hand-forged nails and matching metallurgy. A series of three working boats have been built in original size over the last decade. I will show pictures of the reconstruction phase and the on-the-water tests we performed with different teams over the years to assess speed, maneuverability and sailing performance of the boats. The sailing performance far exceeded the expectations indicating a much larger operating radius for these vessels than previously estimated and thus a much higher flexibility of the river defense scheme that the empire relied on to keep the barbarians at bay. See this movie