The roads to the hinterland of Oberwesel used to lead through the valleys of the Oberbach and the Niederbach. Of the gates that once secured these roads, only the mill gate on the Niederbach has been preserved. In the narrow valley the defense of the town was difficult, because the brook had to be passed through a high and open arch under the town wall. In order to prevent the enemy from entering the city through this arch opening, it was secured by an iron lattice. The foundation of the tower was difficult in the valley bottom. That is why it was built only two stories high. Nevertheless, the foundations had to be secured several times because the stream washed them out. From the battlements of the city wall, one could enter the guardroom above the gateway through a round-arched gateway. From here it was possible to secure the field side through embrasures and the gateway through a cast-iron nose. On the city side, the guardroom was closed by a half-timbered wall. The upper floor was reached by a ladder. This floor had windows on all three sides and an aborter above the stream. The mill next to the gate was the convent mill of the Minorite monastery. Nearby was also the "nun's mill" of the Cistercian convent. The mills inside the city walls were especially important during sieges. A total of 14 mills were still in operation in the Niederbach Valley by the end of the 19th century.