It is walkable if you come out of the Acropolis site at the main entrance off of the Metro station side (the subway). (maybe a mile or so) But if you already have taken the Metro from the port into the city and have the all day train pass (or maybe even if you don't) you can go back into the subway and take the train to the Plaka station..You'll come out across the station from the tomb of the unknown solder, its a closer walk from there.. I'd have to look at the Subway map to give you the correct official names of the stations. You can find it online.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007