Brunelleschi's dome (1418-36) constitutes the first great achievement of Renaissance architecture and engineering and almost 600 years later it remains the largest masonry dome ever built. That is quite an achievement for a man who started his working life as a goldsmith.
Ever since it was erected the dome has dominated the skyline of Florence. And yet, to this day, it remains unfinished!
Filippo Brunelleschi died on April 15th, 1446. In 1460 work began on sheathing the lower part of the octagonal drum with slabs of white and green marble. This was completed in 1503 and, four years later, in 1507, a competition was held for a design for the upper part of the drum, where it meets the cupola.
One of the entrants was Michelangelo, but the winning model was submitted jointly by Simone del Pollaiuolo (better known as il Cronaca), Giuliano da Sangallo and Baccio d'Agnolo. The trio were later joined by Antonio da Sangallo. After the death of Cronaca in 1508 the remaining architects fell out. Guiliano and Antonio da Sangallo resigned leaving Baccio d'Agnolo to continue on his own.
In 1515 he completed the south-east section, but his loggia/gallery attracted far from favourable criticism. Michelangelo, for instance, described it as "gabbia dei grilli" (a cage for crickets). The criticism offended Baccio to such an extent that he downed tools and left the project in a fit of pique!