Fortaleza de la Inmaculada Concepción

The Fortaleza de la Inmaculada Concepción is the reason El Castillo exists. Built by the Spanish in 1675 on a rocky bluff above the Río San Juan, the fort was designed to stop buccaneers and rival colonial powers from using the river as a back door to Granada and the riches of the interior. It worked — mostly. The battlements command a sweeping bend in the river, giving defenders sight lines over the approach in both directions.

The fort was restored in the 1990s and now houses a small but well-curated museum covering the colonial history of the Río San Juan corridor, the pirate raids, and the ill-fated British expedition of 1780 in which a young Horatio Nelson participated. Allow a couple of hours — the museum is more interesting than most at this scale, and the view from the walls over the jungle river is the finest in El Castillo.

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A story
In 1762, British forces seized the fort in a surprise attack during the Seven Years' War — one of the few times the Río San Juan fell to foreign hands. Eighteen years later, a young Captain Horatio Nelson led another British expedition up the river, this time failing to retake it from the Spanish. The fort held, and the route to Granada remained closed to pirates and imperial rivals alike.