Short answers, pulled from the story.
The Regia was a two-part structure in ancient Rome that originally served as a royal palace for the kings of Rome, then became the office of the pontifex maximus, Rome's highest religious official. It stood along the Via Sacra at the edge of the Roman Forum, on a triangular site between the Temple of Vesta, the Temple of Divus Julius, and the Temple of Antoninus and Faustina.
According to ancient tradition, the Regia was built by Numa Pompilius, the second king of Rome, as a royal palace. He was also credited with building the Temple of Vesta, the House of the Vestal Virgins, and the Domus Publica during the same period.
The Regia held the archives of the pontifices, including prayer and sacrifice formulas, the Roman state calendar of sacred days, the Annales (a year-by-year public record of events), and laws relating to marriage, death, and wills. The western room contained the sacred shields and lances of Mars, while the eastern room housed a sanctuary of Ops Consiva accessible only to the pontifex maximus and the Vestal Virgins.
Julius Caesar served as Pontifex Maximus and conducted his priestly duties from the Regia. On the night of the 14th of March 44 BC, the sacred lances of Mars stored in the Regia's western room are said to have vibrated, a legendary omen of disaster. Caesar left the building despite the omen and was assassinated at a Senate meeting that night.
The Regia was burned and restored at least twice in the Republican period, in 148 BC and again in 36 BC. The 36 BC restoration was carried out in marble by Gnaeus Domitius Calvinus, on the original regal foundation. Archaeological layers beneath the site suggest additional earlier versions of the building going back to the Roman monarchy.
The site was first cleared between 1872 and 1875, identified as the Regia by Nichols in 1886, and excavated further by Giacomo Boni in 1899 and by American archaeologist Frank Brown in the 1930s and 1960s. The architectural terracottas from Brown's excavations were published in 1995. Only the foundations of the Republican and Imperial Regia survive today.