The fortification, the Burgus, is presumably a three-storey tower with an area of approx. 20 x 13 meters, which is surrounded by a deep moat a few meters away. The massive stone tower has walls up to three meters thick and is called ''domus fortis supra Lonetam'' - a permanent house on the Lahn - in the Middle Ages. Over the years, this has given rise to the current name of the town of Lahnstein. When the Romans gave up the Rhine Valley in the 5th century, the Burgus lost its military importance. From now on it is used as a residential castle. It is mentioned for the last time in a document in 1549, at that time it is only a ruin. The Roman fortifications were only rediscovered in 1914. Extensive excavations uncovered it in 1926. In order to preserve the important cultural monument, the excavation was shattered again in 1946 with bombs from the Second World War. Since then, the Roman Burgus has been waiting for further exploration.