The Small Defense Tower is located at Hintermauergasse 25 and is a hexagonal, 16-meter high shell tower made of quarry stone masonry. A shell tower means that this tower was made of quarry stone masonry only on the enemy side, and on the city side it was partly made of half-timbering. The tower had a large stone spiral staircase. It served as the main access to the battlements of the town fortification. The staircase was broken out during its later use as a stable and barn.

Owner of the tower until today: 1324-1803 Archdiocese of Mainz, 1803-1840 Duchy of Nassau, auctioned on demolition in 1840 is named as owner of the Fils Tower Joseph Weiland, later probably Johann Zell, from 1890 Jakob Bollinger (according to the city map), until 1955 Maria Schmidt and in 1955 the city of Lahnstein bought it for 800 DM. Since 1980 the tower is under monument protection. In 1976 the then town archivist Willi Eisenbarth took on the tower to preserve it with his two sons and some friends. and to renovate it.

Around 1840, the city wall was demolished to the left and right of the tower. On the town side of the tower, about one meter in front of it, up to the height of the second floor, a new wall with a half-timbered structure was built to have more space for horses and feed. On the left some cross-sections of the tower with the extension visible on the second drawing in the 19th century.