On Mount Oxa (altitude 540.00 m.) a fortified citadel is built, steep from all slopes except the eastern one. The fortress in this strategic position could face any enemy raid effectively, having control over the Gulf of Elounda and the Gulf of Merambello.

The main construction phase of the citadel dates back to the first Byzantine period. In this phase belong the surviving church, with later modifications, several water tanks (cisterns), as well as parts of the fortified enclosure with at least three towers that date to the 7th and 8th century A.D. Few remains from ancient times are preserved as well. The fortified citadel of Oxa probably functioned as a garrison headquarters and as a refuge for the inhabitants of Olous in case of pirate raids. The acropolis seems to have been abandoned at the beginning of the 9th century A.D., when the Arabs seize the island of Crete.

Opposite the citadel of Oxa on the peak "Karfi" (altitude 550.00 m.), quarries of whetstone are found, the "Naxia stone" of antiquity.