The foundation stone was laid in 1156[1] during Vizelin's time as the monastery church of Segeberg Monastery, a former monastery of the Augustinian Canons of Windesheim, which was dissolved during the Reformation. St. Mary's Church is one of the oldest church buildings in Schleswig-Holstein and also the first church with a brick vault. The current church building was first mentioned in 1199 in a document by Pope Innocent III (1198–1216). It was probably not completed until the 13th century.
Around 1470, the church was extended eastward with a choir, and a turret with bells was added. After the monastery was dissolved in 1522 and damage during the Thirty Years' War, these parts of the building fell into disrepair and, along with the northern transept, were demolished. As early as 1573, the retable and altar had been moved to the antechamber, where they still stand today.
The remodeling of the church from 1761 to 1764 was particularly profound: The southern transept was demolished, the church received a large lean-to roof over all three naves, and the main entrance was relocated to the center of the southern nave. From 1864 to 1867, Hermann Georg Krüger restored the church's original form as a basilica with a transept. The tower and the entire church received new walls. The church's current exterior appearance is the result of these measures. Extensive renovations were necessary from 1957 to 1959 because the church was in danger of collapsing. Renovation work has been ongoing since 2010.
Source: Wikipedia