Under construction for over 500 years, the Duomo was built
with 50,000 blocks of marble, ferried on barges from Candoglia
in the Alps, northwest of Lake Maggiore.
This building project spurred the development of the lock. By overcoming a 10-ft. level difference between the moat of the Milan and the Naviglio Grande, the Viarenna lock made possible the delivery of marble directly to the Duomo by barge. The ancillary improvement in commerce benefited the city.
Leonardo contributed ideas for the cathedral and must have been consulted about the Viarenna lock.