The weather-worn, 11th-century, Lombard Romanesque Rotonda di San Lorenzo is sunk below the level of the square, its red-brick walls still decorated with the shadowy remains of 12th- and 13th-century frescoes. The two-level church was 'rediscovered' in 1907 when houses were being demolished on Piazza delle Erbe to make way for a road. This is thought to be the erstwhile site of a Roman temple dedicated to Venus – today's church is still a Dominican place of worship.

　　Narrowly missing destruction during the frenetic Gonzaga refurbishment of the town, this weathered sanctuary once sat within the heart of the Jewish ghetto and its walls are still decorated with the shadowy remains of 12th- and 13th-century frescoes.