The Stiftsruine, also known as the Kleine Marienkirche, is among the most evocative church ruins in western Germany. Bernhard II zur Lippe, the town's founder, donated land here around 1190 for an Augustinian nunnery, and the high-Gothic hall church that grew from those foundations took roughly 150 years to complete. At its peak in 1486 the convent housed 46 nuns; the Reformation dissolved it in 1530, after which it became a secular residence for noblewomen.
A storm in 1819 damaged the roof beyond repair and the building was abandoned the following year. By the 1830s it was structurally unstable. Despite royal intervention the fabric was gradually stripped through the mid-19th century. What remains is a roofless Gothic shell set in the Grüner Winkel parkland, its arched windows open to the sky. The walls are solid and the ruin is visible from the park paths, though access to the interior is restricted.