Mosteiro da Batalha was founded by King João I of Portugal in fulfilment of a vow made before the Battle of Aljubarrota on 14 August 1385 — a battle in which a Portuguese force, outnumbered roughly three to one, defeated the Castilian army of Juan I and secured the independence of the new House of Avis. The king had promised the Virgin Mary that, if granted victory, he would build her a Dominican abbey. Construction began the year after the battle and continued under seven successive Portuguese monarchs for more than 150 years.
Architecturally, Batalha is the most ambitious flowering of Portuguese late-Gothic. The earliest phases — the church, the Royal Cloister, the Founder's Chapel — were directed by the master mason Afonso Domingues from 1388, followed by Huguet, who introduced the more flamboyant tracery and the octagonal Founder's Chapel. Later generations added the Cloister of King Afonso V and, under King Duarte, the rear octagonal chapels that came to be known as the Capelas Imperfeitas — the Unfinished Chapels — when Manuel I redirected royal building funds to Mosteiro dos Jerónimos in Lisbon and the work at Batalha was abandoned with the upper vaults open to the sky.
Inside the Founder's Chapel lie the joint tomb of João I and his queen Philippa of Lancaster — daughter of John of Gaunt and a granddaughter of Edward III of England — and the tombs of four of their sons, including Prince Henry the Navigator. The marriage of João I to Philippa in 1387 sealed the Anglo-Portuguese Alliance, the oldest active diplomatic treaty in the world. UNESCO inscribed the monastery as a World Heritage Site in 1983, citing it as a masterpiece of Gothic art and an exceptional document of late-medieval Portuguese national identity.