Saint Gero Windmill, Architectural heritage monument in Neustadt-Nord, Germany

The Saint Gero Windmill is a 113-metre surviving stretch of Cologne's medieval city wall between Gereonswall and Hansaring, west of the Eigelsteintorburg. Its eastern half-tower, documented since 1446, was converted into a windmill in the 15th century; behind it stands an arched mill passage carried by eight cross vaults.

The wall section and its towers formed part of Cologne's fortifications begun around 1180; the eastern half-tower is documented since 1446 and was converted into a windmill in the 15th century. The tower was repaired in 1558 and 1808, and in 1838 the city sold the mill to Andreas Hochkirchen for 1,460 thalers. The city bought it back in 1908; in 1911–1913 Franz Brantzky incorporated the wall, walkway and tower into the Schnütgen Museum, destroyed in the Second World War.

The names "Gereons-Mühle" and "Herrenleichnam-Mühle" referred to the lost Gereonstor gate and the Herrenleichnam monastery behind it. Since the 1950s the tower has housed the Katholische Studierende Jugend, hence the local nickname "KSJ-Tower".

The wall section runs between Gereonswall and Hansaring, west of the Eigelsteintorburg, in the northern old town. The tower itself is used by the Katholische Studierende Jugend, so it is not open as a museum; the wall, the mill passage and the adjacent Klingelpützpark can be viewed from the public street.

The tower has been used since the 1950s by the Katholische Studierende Jugend, which is why locals call it the "KSJ Tower". The surviving stretch of wall is 113 metres long and once formed part of Cologne's medieval fortifications, begun around 1180.