St. Peter's Church, first mentioned in a document in 1174 as "alde Kerke", is considered the oldest parish church in the city. An archaeologically proven predecessor building can even be dated to the time around 800. The present form, a three-nave basilica built around 1150, whose side aisles were already raised by galleries around 1230, appears almost like a hall church in the interior. The Gothic choir was consecrated in 1322, but was completed much earlier.
Three architectural styles are represented on the church's tower: a Gothic storey was added to the two lower Romanesque storeys at the end of the 14th century and the Baroque (Welsh) dome in 1709. When in 1945 an air mine fell between St. Petri and St. Patrokli, the Gothic choir was almost completely destroyed. Its reconstruction according to the old model until 1955 was practically equivalent to a new building. The squares around the church were cemeteries until the 19th century. The graves still reach in several layers to just below the surface.
A large part of the original ornamental painting of the church, which is the oldest and one of the best preserved examples of a decorative system very common in Westfalen, was uncovered about 30 years ago. The crucifixion group on a pillar of the north wall, created around 1400, is attributed to the painter Conrad von Soest or his school. Also worth seeing are, among other things, a winged altar from Brabant (around 1525), a late Gothic baptismal font (end of the 15th century) and a pulpit with rich figurative carvings (1693).