The Fort of São Teodósio da Cadaveira, located at Estoril, Lisbon District, Portugal, is one of a series of more than a dozen fortresses built between 1642 and 1648 during the period of the Portuguese Restoration War with the purpose of forming a defensive line between the Tower of Belém on the RiverTagus and Cabo da Roca on the Atlantic coast, in order to defend the city of Lisbon. Its construction began in April 1642, and was concluded the following year. Its location permitted crossfire with the nearby Fort of Saint Anthony of Barra to protect the cove of the Poça Beach. It was named in honour of Teodósio, 9th Duke of Bragança, the first-born son of John IV of Portugal.