Various epochs of competing man tend to have a remarkably similar value system when it comes to real estate. One of mans first needs is security and its pursuit is among the most ancient of human endeavors. An area of protection for the family hearth, tribe and social group is necessary to preserve survival, food stores, stored goods, language and culture.
An areas strategic position, its harbors, the availability of water, the arability of its land and its mineral deposits all add to the value of certain locations and make them worth the extra effort to defend and attack.
In Greece, with its successive military rulers, certain areas therefore have both ancient fortifications and medieval fortresses of which some of the later structures are built atop the previous even using pre-existing masonry. Obviously this isn't going to do the earlier structure much good but it was a common enough practice even in Perikles time.
If you can't make it to the Greek islands for lack of time but want to see a great castle, try Naufplion's Palamidi Castle in the Peloponnese only 2 hours by car from Athens. Palamidi is one of the largest, best preserved and best maintained castles in Greece. You can combine Palamidi with two famous nearby ancient bastions, Mycenaea and Tyrins, en route and still be in Athens for a late dinner. Out door sites tend to close at dusk or earlier (7-8 PM in high summer) so an early start is recommended if you want to see all three. Some castles are little more than inaccessible ruins but others such as Palamidi are well worth the effort to visit and old Naufplion a wonderful little town too!