Venice has only one square or piazza: St.Mark’s Square, a 170 meter-long trapezoid which differs from all other squares that are called "campi" (fields).

5 reasons why St. Mark’s Square is utterly unique?

• St Mark’s square has been the heart of the Venetian Republic. The Republic was run by merchant noble families with strict rules, electing institutions and cermonial traditions lasting over than 1.000 years. The Venetian government had its headquarters right here, in the Doge's Palace.
  
  
• Venice’s history is inexorably bound with Byzantium (capital of the Eastern Roman Empire, later Costantinople/Istanbul) for 3 reasons:

1. Byzantium protected and developed Venice as part of its vast empire;
2. Venice conquered Byzantium in 1204 during the fourth crusade, conquering ¼ of its territories. The symbols of Byzantium were looted, carried to Venice and exposed in St Mark’s square: the most important being the 4 bronze horses dominating the Basilica’s façade and the 2 imposing columns in front of Doge’s Palace
3. Istanbul was the capital of the Muslim empire and was as much the enemy of the Venetian Republic as its main commercial partner.

• The Basilica was modeled after the church of Santa Sofia in Istanbul; it was decorated entirely with golden mosaics and marbles of myriad colours and sizes, creating a spectacle that suspends all disbelief.

• The clock tower. The first mechanical clock came to Europe as a donation to Charlemain by the Muslims. From then on, clocks were exhibited as a symbol of state power and efficiency. Venice, following the trend, did the same and built one of the most amazing of clocks of the age, placing right in St Mark’s square of course.
  
  
• Napoleon Bonaparte,was the only one who modified the Square by destroying a marvelous medieval church on the side opposite Basilica to build his own private ballroom; he also stole the Basilica’s 4 horses and the winged lion, symbol of Venice. Fortunately, the Venetians managed to reclaim them.
  

Why St. Mark’s?

Well, St. Mark is one of the 4 official evangelists of the catholic religion, and early on became the patron saint of Venice. His body was smuggled from Egypt and carried into the city in the 9th century AD, since when it remains preserved inside the Basilica. The symbol of St Mark is the winged lion, which quickly became the symbol of the Venetian Republic.