Zarnata Castle is a ruined Ottoman-era fortification about 15 km north of Kardamili, above the small village of Stoupa's inland neighbour Thalames. The castle occupies a commanding position on a rocky ridge overlooking the Mani coast and the Messenian Gulf. The walls are substantially intact, the towers are climbable, and the views from the battlements extend south toward Stoupa and north toward Kalamata.
The castle is not signposted especially well — follow the road toward Thalames from the coast road and look for the ruin on the ridge above. The access track is rough and requires careful driving. The site is unenclosed and free, which means no facilities, no information boards, and no other visitors on most days — which is also its main appeal.