The town of Auch is crossed by the Gers, a tributary of the Garonne which gave its name to the department. The river separates the city into two parts. The upper town, on its left bank, houses the medieval district and most of the ancient monuments. The lower town, for its part, was built on a plain on the right bank. The upper town is connected to the banks of the Gers by typical narrow streets with steep steps from the Middle Ages known as pousterles. A monumental staircase also connects the upper and lower towns through its 374 steps.