Fortress of the Immaculate Conception The Fortress of the Immaculate Conception , ( Spanish : " El Castillo de la Inmaculada Concepción " ) is a fortification located on the southern bank of the Río San Juan ( San Juan River ) , in the village of El Castillo in southern Nicaragua . The fortress is situated approximately 6 kilometers from the border with Costa Rica , at the " Raudal del Diablo " rapids of the San Juan River . It was completed in 1675 as part of a series of fortifications along the San Juan River , to defend against pirate attacks upon the city of Granada ( which can be reached by navigating upstream from the Caribbean Sea along the San Juan River into Lake Nicaragua ) . The settlement of El Castillo and its fortress continued to be strategically important to the Captaincy General of Guatemala until the late 18th century . An important historic landmark of Nicaragua , the fortress is on a list of sites as an initial stage in achieving World Heritage Site status with the United Nations Educational , Scientific and Cultural Organization ( UNESCO ) . History . 16th century . Gil González Dávila was a Spanish conquistador and the first European to arrive in present - day Nicaragua . After discovering the " freshwater sea " ( Lake Nicaragua ) on January 21 , 1522 , he speculated that it discharged into the Caribbean Sea . Francisco Hernández de Córdoba was another Spanish conquistador who founded the city of Granada ( the first Spanish colonial city in Central America ) on the shore of Lake Nicaragua on December 8 , 1524 . Under orders from Hernández de Córdoba , an expedition that included Hernando de Soto , Ruy Díaz ( the first mayor of Granada ) and Sebastián de Belalcázar explored Lake Nicaragua . They succeeded in locating a waterway running eastwards towards the Caribbean Sea , but reported it to be not navigable . Martin Estete , a citizen of Granada , finally discovered the outlet of Lake Nicaragua in 1528 and named it the Río San Juan . Estete was unable to navigate past the rapids however , because the river was low at that time . In 1529 , Diego Machuca became the first man to reach the Caribbean Sea from Lake Nicaragua . After circumnavigating the lake in a group of boats , Machuca and his team entered the San Juan River . From here , Machuca led a group of 200 men who advanced by land , taking the same course as the boats . Machuca named the rapids ( the rapids which stopped Estete the year before ) after himself , and named the port at the mouth of the river San Juan de las Perlas ( later known as San Juan del Norte , and now known as San Juan de Nicaragua ) . 17th century . By the late 17th century , the success of the city of Granada had made it a victim of pirate attacks , the first of which was in 1670 by the pirate Gallardino . In response to this and other pirate attacks , the Spanish colonial authorities constructed the Fortress of the Immaculate Conception as a means to protect the citizens of Granada from future attacks . Captain General Fernando Francisco de Escobedo , who was also at that time Governor of Yucatán , visited the area from January — February 1673 . Escobedo was accompanied by Martín de Andújar Cantos , a military engineer who was responsible for selection of the site . Escobedo and Andújar Cantos arrived at San Juan del Norte , then travelled upstream along the San Juan River and into Lake Nicaragua . Upon their return to Granada , they set about planning the construction of definitive fortifications along the San Juan River . They ultimately decided to build the fortress at the " Raudal del Diablo " , ( known at that time as the " Raudal de Santa Cruz " ) atop the ruins of the previous Fuerte de Santa Cruz , which dated from the time of King Philip III of Spain . Construction of the fortress , initially named " Fortaleza de la Limpia Pura e Inmaculada Concepción " , was begun on March 10 , 1673 and completed in 1675 . Despite the construction of the fortress , buccaneer William Dampier plundered the city and set it on fire on April 8 , 1685 . 18th century . On the morning of July 26 , 1762 , a combined British and Miskito Sambu expeditionary force laid siege to the fortress in what would later be called the Battle for the Río San Juan de Nicaragua . The attacking force consisted of two thousand men and more than fifty boats , while the soldiers at the fortress numbered only around a hundred . The garrison commander , Lieutenant Colonel Don José de Herrera y Sotomayor , had died unexpectedly on July 15 , only 11 days earlier . Inspired by acts of heroism displayed by Herrera ' s 19 - year - old daughter Rafaela ( which included killing the British commander of the expeditionary force ) , " pro tempore " garrison commander Lieutenant Juan de Aguilar y Santa Cruz led the defenders to victory in a battle that lasted six days . The British finally lifted their siege and retreated on August 3 , 1762 . After Spain entered the American Revolutionary War in 1779 , Major General John Dalling , the British governor and commander - in - chief of Jamaica , proposed a second expedition to Nicaragua . The goal was to sail up the San Juan River to Lake Nicaragua and capture the town of Granada , which would effectively cut Spanish America in half as well as provide potential access to the Pacific Ocean . During this expedition , which took place in 1780 and later became known as the San Juan Expedition , Colonel John Polson and Captain Horatio Nelson led a British attack against the fortress . At the time , the Spanish garrison consisted of 228 men under the command of Juan de Ayssa ( who served as Governor of Nicaragua from 1783 — 1787 ) . The expeditionary force succeeded in capturing the fortress on April 29 , 1780 , despite the fact that it consisted of only 200 men . The 22 - year - old Nelson , in command of " HMS Hinchinbrook " , was responsible for leading his men through dense jungle to attack the fortress from a hill in the rear . The British captured the fortress and occupied it for nine months , finally abandoning it in January 1781 . The hill is named " Lomas de Nelson " to this day . 21st century . The Fortress of the Immaculate Conception is a historic landmark of Nicaragua , which currently houses a museum and library . Since 1995 , it has been on a " tentative list " of important Nicaraguan cultural sites with the United Nations Educational , Scientific and Cultural Organization ( UNESCO ) as a preliminary to nomination for consideration as a World Heritage Site .