Sadras is a former Dutch settlement south of Mahabalipuram, where the ruins of an old coastal fort still survive. The site is modest, but it adds a different colonial layer to a stretch of coast better known for Pallava temples.
It is best treated as a short detour if you are already travelling by road between Mahabalipuram and Kalpakkam. Do not expect a large museum or polished visitor centre; the interest is in the surviving walls and the quiet setting.