The Vatican City was founded on the site once dedicated to Nero's Chariot Circus- the Spire stolen from Egypt is the only remaining vestige of that time period

 

After the great Fire of Rome it was the site of many Christian Martyrdoms and according to legend it was in the circus that Saint Peter was crucified upside down. 

 

However, it was considered holy land long before the founding of the first church in the area, and there was a shrine that remained active for a while after that church's founding. 

 

 

 

 

 

 

 

 

 

 

 

 

https://en.wikipedia.org/wiki/Vatican_City#Early_history