Fortezza Medicea di San Martino, Italian fortress in San Piero a Sieve, Italy.

Fortezza Medicea di San Martino is a fortified hill in the Mugello, north of Florence, with about 1.5 km of perimeter walls enclosing 65,000 square metres. Its irregular plan, close to a rectangle, is reinforced by nine bastions, twin at the north-west and south-east, and two gates: Fiorentina to the south and Bolognese to the north. Inside the brick curtain walls stands the five-sided mastio, a smaller fort with its own bastioned walls, troop quarters and a chapel.

Construction began in 1569 under ruler Cosimo I, intended to protect and control the region. Architect Baldassarre Lanci oversaw the building work through its completion in the early 1600s.

The fortress reflects how military architecture was designed during the 1500s, with its two gates named after neighboring cities Florence and Bologna. These names reveal the political importance this location held for the surrounding region.

You can walk on a path that circles the outer walls and view the fortress from different angles. Guided tours into the interior and main tower are available, providing access to areas normally closed to the public.

A secret underground passage ran from the hill down to the Sieve, letting the garrison lead horses to drink during a siege; parts of it can still be traced today. The fortress also held windmills, cannon foundries and ovens, and could shelter up to 2,000 soldiers for months.