Thun's old town occupies a long, narrow island formed by two branches of the Aare as it flows out of Lake Thun. The medieval street plan survives almost intact, with the castle hill at the northern end and the Rathausplatz (city hall square) at the southern. The city hall itself is a massive 16th-century structure that dominates the square.
The most distinctive feature of the Altstadt is the Hauptgasse — the main street runs at two levels, with shops at street level and a raised pedestrian walkway along the first-floor arcades. Residents use the upper level to get from one end of town to the other without crossing the traffic below. The upper walkway has existed since the medieval period; the original wooden arcades were gradually replaced in stone. Two covered wooden bridges over the Aare connect the island to the lakeside and the main railway station.