The area currently known as Turin was once home to a sizable First Civilization city, constructed some time before 75000 BCE. In the center of the city was the Grand Temple, an underground complex that functioned as the focal point in the Vault program. Directly above the Grand Temple stood a colossal statue of the Capitoline Triad: Jupiter, Juno, and Minerva.
After the human rebellion ignited, the Grand Temple was used by Jupiter, Minerva, and Juno to find a countermeasure against an imminent solar flare. Towards the end of their research, Minerva discovered the extent of Juno's fall from grace, following the death of Aita, and had her imprisoned in the Grand Temple.
Ultimately, the trio failed to develop a working solution, and the solar flare struck in 75010 BCE. Turin was completely destroyed, though the underground temple remained largely intact and functional. As the lands changed over millenia, the remains of the Grand Temple became mostly buried under the earth, save for at least one entrance.
During the mid-18th century, the Kanien'kehá:ka nation at Kanatahséton, an area close to Turin, took it upon themselves to protect their "sacred" land from invaders. After having communicated with Juno via the tribe's Crystal Ball, the Kanien'kehá:ka Ratonhnhaké:ton was given the task of joining the Assassin Order to prevent the Templars from entering the Grand Temple. Secretly, Juno's intentions were to have Ratonhnhaké:ton obtain the Grand Temple Key from the Templars, and bury it where his descendant Desmond Miles could find it centuries later.