The rich decorative apparatus comprising hexagonal panels and lozenges embodies the concept of Universal Order and tells the story of the Redemption of Mankind.
The reliefs begin with the Creation of Man and continue with a depiction of his Activities, the Planets which regulate the course of his existence, the Virtues which fortify him, the Liberal Arts which educate him and the Sacraments which sanctify him.
The statues, conceived as an integral part of the building rather than as decorative elements adorning it, deserve individual consideration.
Andrea Pisano replaced the bas-reliefs planned for the second level with sixteen niches designed to contain figures of the Kings and Sibyls and statues of the Patriarchs and Prophets, the latter being carved at a later date by Nanni di Banco and Donatello. They include a splendid group by Donatello depicting the Sacrifice of Isaac, one of the loftiest achievements of 15th century naturalism in the realm of sculpture.
The originals of all these sculptures are now kept in the Opera Museum for conservation purposes.
Work was broken off for two years, between 1348 and 1350, but the bell tower was completed in 1359, after the terrible years of the Black Death, by Francesco Talenti, a talented builder and the designer of the large windows in the upper levels of the tower. We have Talenti to thank for the feeling of airy lightness created by the two-light windows in the Sienese style and the large three-light windows with their tympana, giving us an elegant Gothic building without sacrificing the classical stringency of the original design.
A large projecting terrace at the top, after a climn of over 400 steps, functions as a panoramic roof. It is the final feature in Talenti's design, replacing a spire in Giotto's original plan.