The oldest castle in the town, the Acropolis of Nafplio or Acronafplia, is located on a long, narrow rocky peninsula which enters the Argoliko bay. It measures 85 metres high, 900 metres long and 400 metres wide.

Its-Kale (in Turkish meaning inside fort) as it is otherwise known, was built by the Franks, then passed into the hands of the Byzantines before ending up with the Turks. A hospital and a church were built at the fort during the time of Ioannis Kapodistrias.

The rock of Acronafplia had been inhabited since prehistoric times and was in combat until 1821. Theodoros Kolokotronis was jailed there, as well as many dissidents and political prisoners. Today a luxury hotel stands where the prison was demolished.