Isenburg is best known for the former Isenburg Castle, which today rests as a ruin on a 190m high hilltop, three quarters of which are surrounded by Sayn and Iserbach. An old noble family has been calling itself the Lords of Isenburg since 1095, which gives an indication of the time the castle was built.
The castle complex is located on an area of 70m x 30m and provided space for four residential buildings, which were protected by the probably 25m high keep, gates and bridges. The Romanesque hilltop castle in Sayntal is the ancestral seat of the later county of Isenburg, which comprised large parts of Kurhessen-Waldeck (today's administrative district of Kassel) and southern Hesse (including Offenbach and Neu-Isenburg) and emerged from the various division of family lines. Since the end of the Thirty Years' War, the Isenburg has not been inhabited and has been left to decay, which the Friends of Isenburg, founded in 2005, want to stop with private commitment.
Below the Isenburg Castle was the small settlement that never achieved town charter, but was fortified as a castle town at the beginning of the 14th century. The fortification had four gates, two of which are still preserved. One is the Alte Porz (old gate).
The second preserved gate is the shield gate above the cemetery by the parish church, on the old path to Kleinmaischeid. It is still almost in its original state. It is a square tower with a side length of 4.5 m and a height of 9 m.