Ratnadurga Fort — also called Bhagwati Fort after the temple inside its walls — stands on a rocky headland at the southern end of Ratnagiri town with three of its sides meeting the Arabian Sea. The battlements are largely intact, and the walk along the ramparts gives you a sweeping view of the coastline in both directions, the fishing harbour below, and on a clear day the hills of the Western Ghats inland.
The fort is compact enough to walk around in an hour, and there is no entry fee. Come in the late afternoon for the light and to watch the fishing boats heading back to harbour. The Bhagwati Temple inside the walls is active, so dress modestly if you plan to enter.