Porta Santo Spirito, Rome: View of the unfinished gate

The Sangallo family also worked as military engineers. When Paul III became Pope in 1536, he immediately appointed Antonio (ii) as papal architect. In 1542 he was assigned to concentrate on the defenses of the older Leonine walls. A particularly interesting solution can be seen at the Porta Santo Spirito, Rome, which again failed to reach completion due to disagreements with Michelangelo, who had become the Pope's military consultant in 1545. This architectural solution, even in its incomplete state, would appear to follow the style already visible in the Zecca, showing a 'mature and energetic classicism' (Bruschi).