The roof is open to tourists (for a fee), which allows many a close-up view of some spectacular sculpture that would otherwise be unappreciated. The roof of the cathedral is renowned for the forest of openwork pinnacles and spires, set upon delicate flying buttresses.
The cathedral's five wide naves, divided by 40 pillars, are reflected in the hierarchic openings of the facade. Even the transepts have aisles. The nave columns are 24.5 metres (80 ft) high, and the apsidal windows are 20.7 x 8.5 metres (68 x 28 feet). The huge building is of brick construction, faced with marble from the quarries which Gian Galeazzo Visconti donated in perpetuity to the cathedral chapter. Its maintenance and repairs are very complicated. (text: Wikipedia).