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Photo: DiverDave, CC BY-SA 3.0 via Wikimedia Commons
The fortress is crumbling a bit, but it offers great views of the river and the rapids that made this place a strategic location for a fortress in the first place.
The Fortress of the Immaculate Conception, is a fortification located on the southern bank of the 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. The settlement of El Castillo and its fortress continued to be strategically important to the Captaincy General of Guatemala until the late 18th century.
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Reserva Biológica Indio Maíz
This national park covers a large region ranging from just beyond El Castillo to San Juan del Norte (aka San Juan de Nicaragua) and northwards. Typically it is visited either in a day trip from El Castillo, or by staying at the Refugio Bartola on the edge of the park. There are plentiful guides providing such trips from El Castillo and the Refugio, who will provide you with the requisite rubber boots. Visitors will see verdant and varied rain forest vegetation, and are very likely to also see various types of monkey, leaf cutter ants, poison dart frogs, and plentiful bird life, with other wildlife sightings possible. In March/April 2018 there were significant fires in the region to which the government responded slowly and rejected Costa Rican aid, causing protests which were soon overshadowed by protests against a social security reform which left to several dead and the ongoing (as of July 2018) civil conflict in the country. It is not fully clear how much of the reserve is still biologically intact or what caused the conflagration.