It was begun in 1331 on hill of the Sant'Eufemia by William of Castelbarco, vicar of John of Bohemia, and later completed by Azzone Visconti. A wider citadel was also added, but it is now partly lost. The Venetians built a large tower in the Rocca, as well as a line of walls (Mura Veneziane) 6,200 metres long.