Vatican City as an independent city-state was officially established on February 11, 1929, through the Lateran Treaty between the Holy See and the Kingdom of Italy. However, its history dates far back to the Roman Empire. The area known as the Vatican began as a small plain by the Tiber River, with Vatican Hill becoming important over time. Emperor Caligula built the Circus Vaticanus (a chariot racing stadium) there around 37-41 AD. The site became sacred to Christians as it is believed Saint Peter was martyred and buried there. The first St. Peter's Basilica was constructed beginning in 324 AD under Emperor Constantine over what was believed to be Saint Peter's tomb. This basilica stood until 1506 when the current St. Peter's Basilica began to be rebuilt, a project that lasted until 1626. The walls surrounding what is now Vatican City were built in the 16th century under Pope Pius IV. The Lateran Treaty of 1929 formalized Vatican City as an independent sovereign state with defined territory, including the area within these walls. In summary:

• Area and religious significance: since 1st century AD under Roman Empire
• First St. Peter's Basilica: built starting 324 AD
• Vatican walls: built in 16th century (1500s)
• Vatican City state established: February 11, 1929

Thus, although Vatican City as a state is less than 100 years old, its origins and structures date back nearly two millennia.