Inmaculada Concepción Castle in Río San Juan, as it was known previously, was a Spanish fort, the ruins can be admired and visited today in the margin of the Rio San Juan and facing the stream of Santa Cruz, was sent to built by the Spanish crown in 1672 with the invasion of pirate Gallardillo to the city of Granada.

The construction of Inmaculada Concepción Castle in Río San Juan was assigned to the artillery general Don Fernando Francisco de Escobedo, who was in Guatemala City. The order of construction were done, October 29, 1671, when Madrid.

Construction began in 1673, they chose the more privileged position, a rise next to the most violent rapids of the river San Juan del Norte, in front of Santa Cruz Raudal, which the Indians called the “stream of the Devil”. The work ended in 1675, they baptized the fortress with the name of “Castle of Pure and Clean Conception” in honor of the Virgin Mary.

This place was an important place in the transit route at the junction of Atlantic and Pacific through the San Juan River, Grand Lake, La Virgen (inland port of the lake shore), San Juan del Sur and California, during the gold rush in the nineteenth century .

In military terms, the fortress was defended in 1762 against British troops and Caribbean indigenous called Zambos. In this case it is important to remind the out-standing and heroic action of the young woman of 19 years Rafaela Herrera y Sotomayor who is a national symbol in Nicaragua.

In 1780 another expedition led by the then Captain Horatio Nelson, the same who later became famous in the Battle of Trafalgar, came to El Castillo,. A after an intense combat he took the stronghold, but his men fell by disease and failed.

The fort fell into neglect after the opening of the transcontinental railroad and the construction of the Panama Canal.

The municipality of El Castillo, in the years 1948-1951 and 1968-1970 was a producer of bananas, the product was sold by American companies to Puerto Limon – Costa Rica, the gains were in the hands of large corporations. Following the disease sigatoka, cultivation declined, the large farm did not leave the slightest economic infrastructure.

The ruins of the Castle is visited annually by hundreds of tourists, has oblong figure, was equipped with four bastions, three barracks, a chapel for officers and soldiers, were installed inside domestic services, workshops, hospitals, buildings of poor structure did not withstand the test of time and the destructive action of natural elements such as strength building itself.

The municipality of El Castillo, a member of the political jurisdiction of the department of Rio San Juan, its county seat is Boca de Sabalo, which is located approximately 20 minutes up on the River. The Castle is located 55 km from the provincial capital, is a region remote from major population centers, has a dense vegetation and adverse weather conditions. Historically the town has been for forestry, extract large amount of botanical products for natural medicine and primary wood products. El Castillo shared with the municipality of San Juan de Nicaragua the great Indio Maiz Biological Reserve. The county seat of El Castillo, is located 350 km from the city of Managua, capital of the Republic.