The Grünberg guard tower is part of a medieval fortress in Grünberg.
It was probably built in the 12th century after Mainz troops destroyed half the city including a medieval castle.
After the city and the castle had been rebuilt in 1195, they were additionally fortified with the watchtower. The city was thus extremely well protected for its time.
A special tactic was used in the north: Since there were several monasteries here and an attack on one monastery resulted in the ban of church, attacks from this direction were practically impossible. Attacks from other directions could be quickly detected and repelled from the Grünberg guard tower.
However, even the guard tower Grünberg and the other fortifications could not ward off naked violence, so that the city was pillaged and devastated twice in the 14th century. Although the tower was rebuilt like the entire city, it lost its importance. The Thirty Years' War and a plague epidemic halved the city's population in the 17th century, which also led to Grünberg's decline.
Source: burgen-und-schloesser.net