Where does “ὀχύρωμα” come from?

ὀχύρωμα (Ancient Greek) comes from Ancient Greek ὀχυρόω, from Ancient Greek ὀχυρός, from Ancient Greek ἔχω, from Proto-Hellenic hékʰō, from Proto-Indo-European seǵʰ- — to hold; to overpower.

ὀχύρωμα (Ancient Greek): stronghold, fortress, bastion, citadel, castle (a place that has been fortified)