At the time when Roman civilisation was making itself felt through the rapid construction of roads and buildings, only a few Germanic tribes stubbornly clung on to their marshy territory on the other side of the Rhine, where the city was later to spring up. In the Frankish period of the 7th and 8th centuries, the odd farming or fishing settlement could be found at the point where the small river Dussel flows into the Rhine. The first written mention of the town ' Dusseldorp ' dates back to 1135. Under Kaiser Friedrich Barbarossa the little town of Kaiserswerth, lying at the northern edge of Dusseldorf, became a well fortified outpost of the Empire. From the Palace of Barbarossa, a heavily fortified castle built between 1174 and 1184, soldiers kept a watchful eye on every movement over the Rhine. Kaiserswerth was made into an official district of Dusseldorf in 1929.