In the Middle Ages, the Cistercian Mosteiro de Santa Maria (St. Mary Monastery) was one of the richest and most prestigious in Europe. Begun in 1178, it was founded to honor a vow made by Portugal's first king, Afonso Henríques, before he faced the Moors at Santarém. Alcobaça, at the confluence of the Alcoa and Baça rivers, was built to show his spiritual indebtedness to St. Bernard of Clairvaux, who inspired (some say goaded) many Crusaders into battle against the infidels.

Today, in spite of its baroque facade and latter-day overlay, the monastery is a monument to simplicity and majesty. Above the 98m-long (321-ft.) nave, quadripartite vaulting is supported on transverse arches. These rest on towering pillars and columns. The aisles, too, have stunning vertical lines and are practically as tall as the nave itself.

The transept shelters the Gothic tombs of two star-crossed lovers, the Romeo and Juliet of Portuguese history. Though damaged, their sarcophagi are the greatest pieces of sculpture from 14th-century Portugal. The artist is unknown.

The Cloisters of Silence, with their delicate arches, were favored by Dinis, the poet-king. He sparked a thriving literary colony at the monastery, where the monks were busily engaged in translating ecclesiastical writings. Aside from the tombs and cloisters, the curiosity is the kitchen, through which a branch of the Alcoa River was routed. As in most Cistercian monasteries, the flowing brook was instrumental for sanitation purposes. Chroniclers have suggested that the friars fished for their dinner in the brook and later washed their dishes in it.

Finally, in the 18th-century Salon of Kings are niches with sculptures of some Portuguese rulers. The empty niches, left waiting for the rulers who were never sculptured, lend a melancholic air. The tiles in the room depict, in part, Afonso Henríques's triumph over the Moors.