St. Mark's Basilica is an infrastructure you can purchase for 3 or for 270,000 . The monument will add 12,000 and 900 . It will require 5 , 5 and takes 6 hours to build.
According to the April 16, 2013 update, it is part of the quest "Venetian Cathedral", which becomes available at Level 23. The quest requires the basilica to be built and 300 new residents to be housed.
The building is based on the St. Mark's Basilica, Venice, Italy.