In the 14th century the area comprising Kastel was surrounded by trenches and defensive berms, which were additionally reinforced between 1446 and 1485. In 1497 Berthold von Henneberg, Archbishop of Mainz and Elector, ordered watchtowers built, in response to the many raids perpetrated by bands of bandits. These watchtowers served as an early warning system.
There were originally four such watchtowers, one of which is still standing, near the settlement of Fort Biehler on Boelkestrasse. The so-called “Erbenheim Watchtower” (“Erbenheimer Warte”) is a round tower with a pointed roof and protruding spouts for pouring tar.
Should attackers pose an overwhelming threat, the defenders could retreat to an upper story of the tower, accessible only via a ladder on the outside of the tower. When the ladder was pulled up, the defenders were safe behind the thick wall of the tower. Before retreating, the defenders would signal a warning to the nearby residents, giving them time to grab their valuables and seek safety in the fortified “castel.”