The Four Bronzed Horses
The Four Bronze Horses of St. Mark's, which preside over the whole piazza, are such potent symbols of the pride and power of Venice. Made during the fourth century BCE, these spolia were taken from Constantinople during the Fourth Crusade. They wanted to reflect the prosperity during the transformation of San Marco.  By placing the Four Bronze Horses above the entrance to the Basilica emphasizes the power they have over all that try to conquer Venice and the victory of the Fourth Crusade.
Not only do the Four Bronze Horses of San Marco symbolize victory amongst their enemies, it also symbolizes the Quadriga Domini. During The Middle Ages the quadriga was the subject of a well-known interpretatio christiana, which rendered these four ancient bronzes singularly appropriate for a prominent position on the west front of the church. Although prominent during the time of the Fourth Crusade, many Venetians would not have been acquainted with the quadriga until later.
The Bronze Horses were orginally thought to be completely bronze but after an analysis they have realized that the Horses are 96% copper and 4% bronze.
 Charles Freeman, The Horses of St. Mark’s: A Story of Triumoh in Byzantium, Paris and Venice, (New York: The Overlook Press, 2004), p. 92
 Michael Jacoff, The Horses of San Marco and the Quadriga of the Lord, (Princeton: Princeton University Press, 1993), p. 12