The Little Watchtower is located at Hintermauergasse 25. It presents itself as a hexagonal shell tower made of rubble stone masonry, 16 meters high.

Shell tower means that this tower was made of rubble stone masonry only on the enemy side and partly of half-timbering on the city side. The tower had a large stone spiral staircase. It served as the main access to the ramparts of the city fortifications. The staircase was broken out during its later use as a stable and barn.

Owners of the tower to this day:

1324-1803 Archdiocese of Mainz, 1803-1840 Duchy of Nassau, auctioned for demolition in 1840, as owner of the Fils Tower Joseph Weiland is mentioned, later probably Johann Zell, from 1890 Jakob Bollinger (according to city map), until 1955 Maria Schmidt and in 1955 the city of Lahnstein bought it for 800 DM.

Since 1980, the tower has been under monument protection.

In 1976, the then city archivist Willi Eisenbarth took care of the tower to preserve and renovate it with his two sons and some friends. Around 1840, the city wall was demolished to the left and right of the tower. A new wall with a half-timbered structure was built about one meter in front of the tower on the city side, up to the level of the second floor, in order to have more space for horses and feed. On the left, some cross-sections of the tower with the annex visible in the second drawing from the 19th century.