El Castillo is a municipality located on the Río San Juan in the Río San Juan Department of Nicaragua. The settlement of what would eventually become the municipality of El Castillo began in 1673, when construction of the Fortaleza de la Limpia Pura e Inmaculada Concepción, was begun. The village of El Castillo eventually grew around the fortress, which continued to be strategically important to the Captaincy General of Guatemala until the late 18th century.