This round tower had a diameter of 12 metres and is one of the oldest remnants of Saarbrücken's mediaeval castle. Its stonework consists of so-called convex ashlars, which were used in south-west Germany, especially in the 12th / 13th century. Because the masonry is rather unusual, it is assumed that the stones were originally used elsewhere and later re-deployed for the Red Tower. Thus there is some controversy as to the exact dating of the tower.