In the southern suburb, the city fortifications on the Rhine side were secured by two round towers; on the one hand by the Red Tower and on the other hand by the tower Haus Schönburg. The name of the tower already indicates that the town palace of the Schönburgs was once located in its area. Since the round tower is located in the middle of the city wall, the battlements run through the middle of the tower. Even today, the round-arched battlements can be seen on both sides. The diameter is almost six meters. At a height of nine meters, the tower is also smaller than the other towers on the Rhine side of the city fortifications. On the upper floor or on the defense platform, the tower is lined by a round arch frieze made of light-colored sandstone and a crenellation with chamfered embrasures above it. Also conspicuous are the pointed-arched windows inserted later on the Rhine side and the octagonal pewter rim of the observation platform, which is connected above the round-arched frieze on sand-lime stone consoles.