Forte di Belvedere is a star-shaped fortress built for Grand Duke Ferdinando I between 1590 and 1595, its design attributed to Bernardo Buontalenti. Perched on the ridge above the Oltrarno with the Boboli Gardens falling away below, it commands what is arguably the finest panoramic view of Florence: the Duomo, Palazzo Vecchio, and the Arno laid out across the valley, framed by the surrounding hills.
The fort served as a treasury and private escape for the Medici, connected directly to the Palazzo Pitti by a covered walkway. Today it opens for major temporary exhibitions (often contemporary art and sculpture using the ramparts and open-air terraces), but the best reason to climb up is simply the view. Access is via the steep Costa San Giorgio or from the Boboli Gardens. Check ahead for current opening — when no exhibition is scheduled, the ramparts may still be accessible.