Schnetztor is a compact survivor of Konstanz's medieval defences, a stone gate tower where the old road entered the town from the south. Its steep roof, small windows, and thick lower walls make it a good visual marker between the shopping streets of the old town and the quieter lanes beyond.
You do not need to plan a long visit around it. The gate works best as part of a slow walk through Konstanz: pass under it, look back at the tower from the street, then continue toward the cathedral, harbour, or Niederburg quarter. It is a small sight, but in a city where much of the pleasure is in the old urban fabric, it helps fix the shape of the former walled town.