The "Old Tower" is a remnant of the old town church
"Zu unserer lieben Frau", which was built around 1360 as a chapel dedicated to the Virgin Mary within the town walls, extended in 1401, given a tower and choir and presumably elevated to parish church status soon afterwards.
When the old church was demolished after the completion of St John's Church in 1876/77, the church tower remained standing; the double lantern in its spire is characteristic.
The layout of the old church can be seen from the paving of the pedestrian area at the foot of the "Old Tower".