On the 26th December 1476, the Lord of Milan, Galeazzo Maria Sforza, son of Francesco Sforza and Bianca Maria Visconti, fell victim to a plot and was stabbed to death, causing his wife Bona di Savoia, to move to the safest place within the castle and further fortify it with the addition of a tall tower. The Bona Tower, at the intersection of the north-east and south-east wings allowed for control of the entire building. In addition to a defensive function, the tower was also used as a prison, as is testified to by a door equipped with a spy-hole, which can still be seen today in the stairwell. Damaged by ensuing foreign dominations, the tower was finally restored as part of the works initiated in 1893 by Luca Beltrami, who also raised it and added battlements.