Prosek, also known as Stenae, is the old fortified settlement in the canyon above Demir Kapija. The remains are fragmentary, but the position explains everything: this was a strategic hold over the Vardar passage, with towers, ceramics, coins, and burial finds uncovered in the area.
Go for the sense of place rather than clean ruins. The best part is seeing how the old stronghold used the cliffs and river corridor.