"Containing the sacred fire and the Palladium, an effigy of Athene (Minerva) believed to have been brought by Aeneas from Troy, this ancient temple was built in imitation of a primitive round hut, its hearth fire symbolizing the perpetuity of the Roman State. It was not a true temple in that its space was not inaugurated, nor did it contain an image of Vesta, the goddess of the household hearth. As the handmaidens of Vesta, the principal duty of the six Vestals was never to allow the flame to be extinguished, an arduous task in a building with a vent in the roof. There also was danger that the temple, itself, might catch fire, which it sometimes did. It was destroyed in the fire of Nero in AD 64, which reached this point of the Forum. The last time it burned, in AD 191, the temple was restored by Julia Domna, the wife of Severus. Once a year, on June 15, the ashes of the tended fire were ritually thrown into the Tiber.
Completely stripped of its marble in the mid-sixteenth century, a section of the temple was reconstructed in 1930." - Encyclopedia Romana, University of California at San Francisco.