Monza is crossed from north to south by the River Lambro. The river enters Monza from the north, between Via Aliprandi and Via Zanzi streets. This is an artificial fork of the river, created for defensive purposes in the early decades of the 14th century. The fork is known as Lambretto and it rejoins the main course of the Lambro as it exits to the south, leaving Monza through the now demolished ancient circle of medieval walls. Another artificial stream is the Canale Villoresi, which was constructed in the late 19th century.