Vatican City, as a country , was not exactly “built” at one time – it was formally created as a sovereign state in 1929 with the Lateran Treaty between the Holy See and Italy.

If you’re thinking in terms of buildings:

  • The area became an important Christian site in the 4th century, when Emperor Constantine built the first St Peter’s Basilica around 319–326 over what was believed to be the tomb of Saint Peter.
  • The current St Peter’s Basilica was constructed much later, roughly from 1506 to 1626.
  • The Sistine Chapel was built between about 1473 and 1481, and Michelangelo painted its ceiling from 1508 to 1512.

So, in everyday terms:

  • “Vatican City” as a state dates to 1929.
  • Its key religious buildings and walls were built gradually from the 4th century through the 16th century.