The Montemor-o-Velho Castle is the biggest medieval fortress existing in the Mondego River area. It is a moist ancient ensemble, with Roman traces detected in its perimeter.

In the 9th century, the Montemor-o-Velho Castle was already in possession of the Asturian Kingdom. In the 10th century, momentarily under the Moorish rule, it underwent some constructive works by Ben-Alfagi's command. During the following centuries, it was successively lost and conquered again following Coimbra's military fate. It was only with the final conquest of the town, in 1064, by King Fernando, O Magno (The Great), that the Montemor-o-Velho Castle underwent deep alterations ordered one century later by King Afonso Henriques, who made Coimbra the reign's capital. From that early period nearly nothing remains (maybe the lower ashlars in the donjon can be assigned to the High Middle Ages)

Its today's configuration is due to a total reform made in the 14th century (1331). A palace to the princesses was built then, which was transformed over and over again till the Manueline period. The donjon topped with pentagonal battlements and an upper floor surrounded by huge stones and lengthy brackets were also built, dating back to that period (King Afonso IV's campaign).

Inside the Castle, there is the Igreja de Santa Maria da Alcáçova (St. Mary of Alcáçova Church), a Gothic temple of the 14th century, with three naves and tripartite chancel, rebuilt later by the Manueline architect Marcos Pires, from Coimbra. Some art pieces are noteworthy: the Gothic limestone sculptures of Virgem do Ó and Anjo da Anunciação, masterpieces of the 14th century made by Master Pero; a set of Hispanic-arabian glazed tiles dated to around 1500. The Montemor-o-Velho Castle is a true landmark on the valley due to its privileged location. The restoration works that have been taking place for the last decade favoured the structural consolidation of the walls, the gardening of the yards and the general electrical installation. On a second phase, the interior of the Castle underwent major reforms in order to welcome the visitors.