La laguna fortificada de aooot raiti bocay, Spanish colonial fortress in El Castillo, Nicaragua.

The Fortress of the Immaculate Conception stands on the southern bank of the San Juan River at the Raudal del Diablo rapids, about 6 km from the Costa Rican border. Construction began on 10 March 1673 on the ruins of the earlier Fuerte de Santa Cruz and was completed in 1675. It was first named Fortaleza de la Limpia Pura e Inmaculada Concepción.

A Spanish military engineer completed construction in 1675 to defend Granada against pirate raids coming through the river. The fortress became the main defense post for Spanish holdings in this part of Central America during the colonial period.

The village and fortress take their name from the image of the Immaculate Conception kept in the castle chapel. Tradition holds that during the 1762 siege a sparrow flew into the chapel and sang before the image, and the besiegers withdrew the next day; the defenders read it as a miracle. The hill from which Nelson's men attacked is still called Lomas de Nelson.

Visitors reach the site by motorboat from San Carlos, and the journey follows the river course to the fortress entrance. The structure has stairs and uneven stone surfaces, so wear comfortable walking shoes and be prepared for humid conditions.

A British naval officer captured the fortress in 1780 during his early military career, an event that highlighted the site's strategic value. This episode shows how European powers competed for control of Central America's most important waterways.