Pomerium

The Pomerium was a religious and sacred boundary around the Ancient City of Rome and other Roman cities in the Roman Empire.
It was expanded with each conquest of a new Province.

The Pomerium was not outlined by a wall, such as the Servian Wall but followed a course both inside and outside them. instead it was defined by white marker stones called Cippi.
Roman tradition believed the first boundary of the city of Rome was marked out by Romulus with a plough on the day Rome was founded, the 21st April 793 BCE.

It was forbidden to bury the dead inside the Pomerium, which is why Roman Cemeteries are along roads outside of Rome, and of all Roman towns.

Generals and Promagistrates forfeited their Imperium on entering the Pomerium. And they were forbidden to enter the Pomerium with their Armies. Victorious Generals celebrating their conquests, had to wait outside, then enter as a civilian and their troops likewise.
For this reason the Praetorian Guard were not allowed to wear military uniform or weapons inside the Pomerium, and had to wear civilian dress, the Toga. They were known as the Cohors Togata.

In 88 BCE, Lucius Cornelius Sulla was the first to break this law when he took six of his Legions and entered the Pomerium forcing Marius to flee and the Roman Senate to agree to his terms.

In 49 BCE, Julius Caesar broke the law again by Crossing the Rubicon, which was by then considered to be the extended territory of the city of Rome.

An intact Cippi, the large white marker stone, has been excavated and carries an inscription dating it to 49 CE.
It is now on display in the Ara Pacis Museum.