Fortín de San Gerónimo Fortín de San Gerónimo del Boquerón ( " Fort Saint Jerome of the Large Entrance " ) is a small fort located in the entrance to what is known today as the Condado Lagoon which faces the historic town of Miramar in San Juan , Puerto Rico . It was built during the 17th century to replace a smaller battery ( called El Boquerón ) that stood at the easternmost end of the San Juan islet . The original Boquerón battery defended San Juan from attacks by Sir Francis Drake in 1595 and George Clifford , the third Earl of Cumberland , in 1598 who destroyed it after his attack . The San Gerónimo became part of San Juan ' s First Line of Defense , along with the San Antonio Fort / Bridge and Escambrón Fort , being the Fourth and final Line of Defense the majestic San Cristóbal Castle , guardian to the city entrance . The fort is still in remarkably good shape , is open for visitors and has access to the Caribe Hilton Hotel and Plaza Caribe complex . Overview . The San Gerónimo and San Antonio were pivotal in repelling the invasion by the forces of British Admiral Henry Harvey and Sir Ralph Abercromby in 1797 . On April 17 , 1797 their combined forces ( 64 to 68 ships and 7 , 000 to 13 , 000 men ) landed in Cangrejos , approx . 3 miles from San Juan . San Juan ' s military forces which included , among others , the Regimiento Fijo de Puerto Rico and the Milicias Disciplinadas were outnumbered by the invaders almost 3 to 1 . Admiral Harvey blockaded the San Juan Harbor while Abercromby established his operations headquarters in San Mateo parish overseeing all of San Juan and the Martín Peña Bridge . Abercromby ' s strategy was to take the Martín Peña bridge in order to block Spanish reinforcements from the South and bombard San Gerónimo and San Antonio from Miramar to gain access and cross the San Antonio Bridge into the San Juan islet . British forces included the Royal Marines made famous by later on defeating Napoleon ' s troops in Egypt . Governor Ramón de Castro , a brilliant strategist prevented the British from advancing and eventually conquering San Juan . The San Gerónimo and San Antonio , sustained heavy damage from the constant bombardment during the two - week siege on the city . Chronicles have the defenders shooting their muskets and cannons from behind sandbags because of the devastation their forts sustained . The San Gerónimo and San Antonio were eventually rebuilt and expanded ; there is some information that says they were rebuilt to look as they did prior to the attack . San Gerónimo is located adjacent to the grounds of the Caribe Hilton Hotel . It was listed on the National Register of Historic Places on October 11 , 1983 . Unlike similar forts near Old San Juan , it is not part of San Juan National Historic Site . San Gerónimo is owned by the Institute of Puerto Rican Culture , but is managed by the Hilton . The hotel occasionally used it for private gatherings , but during recent times it has fallen into such disrepair that visits are no longer allowed . The " Polvorín de San Gerónimo " or gunpowder house that was built in 1769 and supplied the fort with gunpowder is now part of the Luis Muñoz Rivera Park . As recent as the year 2006 attempts are being made by the Instituto de Cultura Puertorriqueña and the U . S . Army Corps of Engineers to build a breakwater to stop any further deterioration from irreparably damaging the centuries old fort . Senate Vice President Orlando Parga has led efforts to modify some of the construction projects that would encroach on the fort or would hamper the Corps of Engineers preservation and reconstruction efforts . In July 2007 , the Fort was the scene of protests against the construction of a tourist project called Paseo Caribe that would block access to the fortification . Protesters argued that the public should have free access to it . The protests lasted about a week during which the manifestants halted the construction projects by climbing into the cranes . The Omnibus Public Land Management Act of 2009 directs the U . S . Secretary of the Interior to conduct a study to determine the suitability of including San Gerónimo as part of San Juan National Historic Site .