The Sassenpoort is Zwolle's great medieval survivor, a brick gate with two round towers and enough mass to make the old city boundaries feel real. It stands just south of the centre, where you can still sense the transition from defended town to later city.
Even if the interior is closed, the exterior is worth the walk. Come in the late afternoon if you can, when the brickwork catches the light and the gate looks less like a monument and more like a working piece of city fabric.