The Portico of San Luca, the world's longest portico, was built with 666 arches to specifically protect the Byzantine icon of the Madonna that supposedly stopped the rain from ruining the crops in the 15th century. The people prayed to it to stop the rains, and when the icon entered the city, the rains immediately stopped. Had Clarkson been able to complete his lecture, he likely would have said that the meaning of the 666 arches has been debated for centuries. The leading theory is that having the portico shaped like a snake and having a devilishly-number of arches is supposed to signify the Madonna's victory over evil.
Clarkson has Jeremy tell the waiter "I am a bellend," which is of course a rude joke for the British.