Bierbaums Nagel, Observation tower in Willebadessen, Germany

Bierbaums Nagel is a stone observation tower standing 13.5 meters tall in the forested landscapes of Willebadessen. From its platform, visitors see out across the wooded hills and valleys that make up the surrounding region.

Julius Bierbaum built this structure in 1849 to create work for the local population and support the rural economy. The building became a landmark reflecting both his charitable intentions and the industrial efforts of the era.

Julius Bierbaum (1817–1865), son of the merchant Julius Georg Bierbaum, owned Schloss Borlinghausen from 1839 to 1859. The tower's name refers to its shape: it rises from the landscape like a huge nail. His wife Catharina Margaretha Franziska Rommel, born in Kassel, rode up on a donkey, so the approach is still called Eselspfad.

The tower stands on a 431.4 m high hill in the Borlinghauser Holz forest, about 1.5 km west-northwest of Borlinghausen. A short spur path branches off the shared route of the European long-distance trail E1 and the Eggeweg, which passes about 200 m west of the tower. The lower part of the tower is fitted out as a shelter.

The tower was originally built with Kassel in mind, since Bierbaum's wife came from that city and felt homesick. On clear days, she could spot the Herkules monument in the distance from the platform, which helped ease her sense of isolation.