Protecting the centre of the Alentejo town of Alter do Chão, the Castle is a fine example of fourteenth-century medieval architecture, erected during the reign of Dom Pedro I, in 1359. At that time, it constituted a defensive articulation with Estremoz, Fronteira, Évora Monte, Avis, Alter Pedroso, Marvão, and Portalegre. King D. João I, Master of Avis, made Nuno Álvares Pereira donatory of the town. Inherited by the Constable of the Kingdom’s daughter, it became part of the domain of the House of Braganza in the 15th century and was then transformed into a fortified manor house. King D. Fernando II used it as a prison. The castle has an irregular layout and draws a pentagon. The high wall is reinforced by three rectangular towers. Further back, there is the high, quadrangular keep with its 44 metres of height. The Alter do Chão Castle has been classified as a National Monument since 1910.