The Palazzo Pretorio is the building that commands Certaldo Alto — a solid, crenellated fortress originally built in the 12th century for the powerful Alberti Counts and later adapted as the seat of the Florentine Vicariate that governed the surrounding territory for centuries. From its tower you get the clearest view over the valley; from the square in front of it, you understand why the Alberti chose this hilltop.
Inside, the palace functions as a museum covering the history of Certaldo and the Vicariate. The highlight is the adjoining deconsecrated church of Santi Tommaso e Prospero, which preserves frescoes and sinopites — the preparatory drawings that underlie finished fresco — from the 15th and 16th centuries. These sinopites, normally hidden beneath layers of plaster, survived because the outer frescoes were detached for conservation, leaving the rough red-ochre sketches visible. They are unusually expressive, and seeing the artist's working drawing alongside the finished image gives you a rare sense of the craft involved.