CASTRO Marin, a port in the province of Algarve, in the kingdom of Portugal. It is situated at the mouth of the Guadiana, the boundary betwixt Spain and Portugal, and is not more than two miles from Ayamonte, in Andalusia. This town contains 450 houses, and 1800 inhabitants, many of them employed in fishing and in a smuggling trade. Lat. 37. 11. N.