The really remarkable part about the Parthenon is that it likely would have been almost completely intact, if it hadn't been used as a powder magazine and blown up in the 1800s. It wasn't gradually ruined, it just blew up relatively recently (historically speaking).
All correct, except for the date, which was 1687 [1]. It happened during a war between the Venetians and the Ottoman Turks (the Turks stored gunpowder in the building; the Venetians fired a mortar at it). The Venetians compounded the damage by their incompetent attempt to loot some sculptures from the west pediment; these fell to the ground and smashed to pieces.
