The port of Nafplio in the Peloponnese is an absolute must see for the military history buff. It has three castles and a very pretty town to keep the rest of the family entertained.
In ancient times the town was under the control of Argos as its main port, until the the Byzantines used it as a base for their anti-piracy operation. Then the Franks held it for around 180 years before giving way to the Venetians. In 1540 it fell to the Turks after a three year siege, although the Venetians briefly grabbed it back between 1686 and 1715. It was the first capital of independent Greece in 1827.
The main castle is the Palimidi which towers above the town. When you enter the main gate the Venetian lion immediately tells you who built it. It has five substantial bastions and is a huge site, so allocate a bit of time. If you are feeling fit there is a 999 step staircase from the town, but mere mortals like me drove up to the main gate on a surprisingly good road.
The Akronafplia castle defends the eastern approaches to the town. This was the Byzantine and then Frankish castle although improved by the Venetians. You can drive right through the site, or use a lift from the east end of the town.
Finally, we have the small island fort of Bourtzi, which defends the harbour. It was built by the Venetians at the end of the 15th century. A boat trip will take you out to the castle, but it is currently being renovated - very slowly!