Consider the following.
In 73BC a slave rebellion erupts in Capua in a gladiator school. This soon turns into a real war between Rome and a slave army led by Spartacus. This war wil rage for two years. It is finally ended by two men who will later play a very important role in Roman politics: Pompeius and Crassus.
Now, Iulius Caesar was also part of this piece of history. He was back in Italy, serving a term as military tribune, and most likely in Crassus’ army.
Now imagine the following.
Caesar, ambitious visionary as he was, seized an opportunity. When part of the slave army split off he made a deal, in secret. Instead of crushing them, he let them live on land belonging to him. They lived in an abandoned Etruscan city until such time when their service would be needed.
Sixteen years after this war Caesar was appointed proconsul of Gaul and he started his conquest. This is the time when he called in his favor.
This is the story of the origin of the Praetoria Prima.