By order of the master of the Order of St. John, Gregory of Schabendorf, in 1491 the construction of town walls had begun. The 1400 metre long wall was made of field stones and bricks.
In the western part, on marshy land near the mill pond, a palisade was built. There was no guardhouse in the wall, but in its north-western part a round tower was built. There used to be two gates in the wall: from the southern direction one could enter the town through the Ostrów Gate (which is said to have been a high tower with a steep ridge roof), and from the northern direction - through the Mill Gate bridge over a moat in the intramural space. Inside the walls, near the Mill Gate there was the Salt House.
Only two fragments of the northern part of the town walls of a total length of about 200 metres have survived. A defence outlet and a gate bricked up in the shape of an arch are also partially visible. Between the Postomia River and the wall there is the oldest building in town - the House of the Order of St. John.