El Castillo is a municipality located on the Río San Juan (San Juan River) 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, (now known as the Castillo de la Inmaculada Concepción, or Fortress of the Immaculate Conception) was begun.