Just 10 kilometres (6.2 mi) from Portalegre, the castle of Alegrete is one of the more important fortress in the Alto Alentejo, assuming since the Middle Age, a fundamental position in regional defense.

[1][2] The first documents suggest it dated from the 13th century, during the reign of King D. Afonso III, when the fortifications were either constructed or reconstructed.

[1][2] The perimeter of the fortification was quite irregular, and did not adopt the oval plan generally used during the Portuguese Gothic period.

[1] Meanwhile, though, the dynastic crisis resulting from the death of King Sebastian at Alcácer Quibir, revitalized the importance of the medieval castle, owing to its frontier position along the Spanish border.

[1] At the end of the 17th century, King D. John IV compensated General Matias de Albuquerque by making him the Count of Alegrete, and putting the castle on the first line of defense during the Portuguese Restoration War.

[1] Within twenty years, the conflict between Absolutists and Liberals erupted within Portugal, leading to combat within the castle's proximity, between troops of the Count of Vila Flor and royalist forces.

[1][2] The urban fortifications, with castle to the south, delimit the primitive settlement on top of the mountain, at about 502.9 metres (1,650 ft) altitude.

In the south-southwest of the castle are the remains of a cubel/watchtower, with a rectangular surface, that accessible from a staircase addorsed from the southern wall.