St Mark’s Basilica (Basilica di San Marco) is Venice’s principal church and a symbolic statement of the city’s wealth, power, and Mediterranean trading network; it began as a small shrine to house the relics of St Mark in the 9th century and was rebuilt in its present, richly Byzantine-influenced form in the 11th century, consecrated in 1094.
 
The basilica’s origin ties to the arrival—traditionally described as theft—of St Mark’s relics from Alexandria in 828, which replaced St Theodore as Venice’s patron and helped legitimize Venice’s civic identity.
 
Many visible elements are spoils or imports—classical columns, porphyry statues (notably the Tetrarchs) and the famed Horses of St Mark, brought from Constantinople after the Fourth Crusade (the horses on the façade today are replicas; the originals are kept inside).
 
I have also painted a detail of the facade, check it out.
 
Interesting facts...
look around for oddities such as later additions and graffiti-like marks, a modern re-used relief with a bullet‑mark or gun depiction added by 19th‑century sailors after an Austrian bombardment.