The original construction was ordered by local lord Galeazzo II Visconti in 1358-c. 1370; this castle was known as Castello di Porta Giova (or Porta Zubia), from the name of a gate in walls located nearby. His successors Gian Galeazzo, Giovanni Maria and Filippo Maria Visconti enlarged it, until it became a square-plan castle with 200 m-long sides, four towers at the corners and up to 7 m-thick walls. The castle was the main residence in the city of its Visconti lords, and was destroyed by the short-lived Golden Ambrosian Republic which ousted them in 1447.