Arnolfo di Cambio
The architect is said to be Arnolfo di Cambio, who used Poppi as a 'prototype' for the Palazzo Vecchio, which he also designed. The two buildings certainly do look similar, and originally the tower of Poppi would have been taller, with machicolations (projecting parapets with 'murder holes' for dropping various missiles on the enemy beneath) that would have made the appearance even more similar.