Ancient Acro-Corinth wasn't high on my lists of must sees but that was a mistake. The drive up the hill is well worth it because the views are amazing. Its only 2 hours from Athens and easy to get to and great place for a picnic. Its so big up top you can get lost... I did.
Its one of the best fortified hilltops in Europe at least from a height perspective. Its history is long but don't expect a lot in the way of preservation of the really ancient fortifications. The medieval ruins are more extensive and are of Venetian, Norman and Turkish defensive construction. Even the Nazi's occupied it. For a lot more on Acrocorinth click here.