The severe west front is relieved by three rows of semicircular arches, and has a central porch (there were at one time three) supported by huge red marble lions, sculptured no doubt with the rest of the façade by Giovanni Bono da Bissone in 1281.
A semicircular or semi-elliptical pit (calcarone) about 33 ft.
In the Russian Orthodox Church the term "ambo" is used of the semicircular steps leading to the platform in front of the iconostasis, but in cathedrals the bishop has an ambo in the centre of the church.
Outside this again lies a projecting semicircular bastion, which commands the entrance from the exterior of the city on the E., a winding trench approached by a pair of double gateways, which are not vaulted but covered by the gradual projection of the upper courses.
The plan consists of a large rectangular nave, with semicircular recesses for altars, opening out of the aisles, north and south.