The first endeavour to build a sanctuary for Athena Parthenos on the site of the present Parthenon was begun shortly after the Battle of Marathon (c. 490-88 BC) upon a massive limestone foundation that extended and leveled the southern part of the Acropolis summit. This building replaced a hekatompedon (meaning "hundred-footer") and would have stood beside the archaic temple dedicated to the Athena Polias. The Older or Pre-Parthenon, as it is frequently referred to, was still under construction when the Persians sacked the city in 480 BC and razed the Acropolis.
(Taken from Wikipedia)