While the Piazza del Duomo is the most important square in central Milan today, that title went to the Piazza Mercanti in the Middle Ages. The Piazza Mercanti, or Merchants Square, is in Milan's historic center, a short walk to the northwest of the Duomo. The square originally was much larger than it is today, once occupying part of present-day Via Mercanti. Some of the buildings on the square date from its heyday, including the 13th century Palazzo della Ragione (essentially Milan's city hall at the time) and the 14th century Loggia degli Osii (another administrative building).
In the center of the piazza is a 16th century well that was later fitted with two columns in the 18th century. It's covered and unused today There was a stone found nearby upon which merchants found guilty of cheating – or bankruptcy, depending on the story you read – were punished by public shaming, forced to stand on the stone with their pants down, before being sent to jail.
To reach Piazza Mercanti, walk from Piazza del Duomo's northwest corner along the angled street Via Mercanti. You'll see the piazza on your left in just a few steps. It's between the Metro stops of Duomo and Cordusio.