Alcobaca is chiefly interesting for its Cistercian convent, now partly converted into schools and barracks. The monastic buildings, which form a square 725 ft. in diameter, with a huge conical chimney rising above them, were founded in 1148 and completed in 1222. During the middle ages it rivalled the greatest European abbeys in size and wealth. It was supplied with water by an affluent of the Alcoa, which still flows through the kitchen; its abbot ranked with the highest Portuguese nobles, and, according to tradition, 999 monks continued the celebration of Mass without intermission throughout the year.
The convent was partly burned by the French in 1810, secularized in 1834 and afterwards gradually restored. Portions of the library, which comprised over 100,000 volumes, including many precious manuscripts, were saved in 1810, and are preserved in the public libraries of Lisbon and Braga. The monastic church (1222) is a good example of early Gothic, somewhat defaced by Moorish and other additions. It contains a fine cloister and the tombs of Peter I (1357-1367) and his wife, Inez de Castro.