The Citadel of Cascais is a set of fortifications built between the 15th and 17th centuries to defend the Cascais coastline and River Tagus estuary and to protect against attacks on the capital of Portugal, Lisbon. The citadel incorporates three separate developments, the tower of Santo António de Cascais, the Fortress of Our Lady of Light, and the former Royal Palace area. It serves as an official summer residence for the President of Portugal.