The Río San Juan was once critical to the entire region, the river was the main route of access to the entire country. Wide and navigable the river allowed hundreds of miles of thick tropical jungle to be easily transited. People and cargoes could be shipped from the Caribbean to the enormous Lake Nicaragua and the rich farmlands of central Nicaragua. Together the river and lake offered an easy crossing of the isthmus, allowing passengers and cargo to pass between the Atlantic and Pacific Oceans. At the northern end of Lake Nicaragua is Grenada, this old colonial city was the crown jewel of the region, where the treasures of the New World were amassed before shipment to Spain.
Defense of this critical river link proved to be necessary in the tumultuous history of the region. On several occasions pirate forces passed this way to loot and burn Grenada. In response to these incursions the Spanish selected a site on a high bluff and overlooking a significant set of rapids on the river as the ideal site for a defensive fortification. A large stone fort was built here, completed in 1675, becoming the lynch-pin for defending this critical access route to the interior of Nicaragua.
Somehow the name Fortress of the Immaculate Conception just does not bring to mind any form of military installation. Despite the name, this fortress is an impressive fortification, easily equivalent to the best contemporary fortifications elsewhere, quite a surprise as the location is and was quite remote. Most photographs fail to convey this, much of the fortress walls are concealed by a surrounding ditch making it difficult to appreciate the fortress without a visit in person. Setting the walls low like this was an important feature of fortifications built during the age of cannon, making the walls a difficult target for attacking gunners. The defenses are well laid out, clearly the work of an experienced military architect. The bastions are properly angled to deflect incoming artillery rounds, gun-ports are positioned to sweep the walls of attacking infantry. Taking this position from prepared defenders would be a difficult proposition indeed.