The architecture is a synthesis of the Romanesque and Gothic styles, and established many of the typical characteristics of Italian Gothic architecture. As originally built, both upper and lower churches had a simple cruciform plan with an aisle-less nave of four square bays, a square crossing, a transept that projected by half a bay on each side, and an apse, the lower being semicircular and the upper polygonal. To the left of the church stands a free-standing bell tower of Romanesque design.