The cubical building is made of solid rusticated stonework, with two rows of two-lighted Gothic windows, each with a trefoil arch. In the 15th century, Michelozzo Michelozzi added decorative bas-reliefs of the cross and the Florentine lily in the spandrels between the trefoils. The building is crowned with projecting crenellated battlement, supported by small arches and corbels. Under the arches are a repeated series of nine painted coats of arms of the Florentine republic. Some of these arches can be used as embrasures ("spiombati") for dropping heated liquids or rocks on invaders.