A vast 15th-century red-brick fortress built by the Sforza duchy, now housing several civic museums. The most important is the Museo d'Arte Antica, which contains Michelangelo's unfinished Pietà Rondanini — his last work, left incomplete at his death in 1564, and one of the most moving sculptures in Italy.
The castle grounds are free and open to the public, connecting to the large Parco Sempione behind. Worth entering even if you skip the museums; the courtyard gives a good sense of the fortress's scale.