The Castle of Monsanto site was likely the location of fortified settlement during prehistory. Legend suggests that during the 2nd century a castro was taken and colonized on the hilltop of Mons Sanctus, by Roman Praetor Lucius Aemilius Paullus. The castro was destroyed and progressively settled by settlers from Idanha-a-Velha.
Between the 5th and 11th century was occupied by Visigoths and Arabs. The castles of Monsanto and Idanha-a-Velha were constructed by 1171. A contemporary castle was constructed during the 13th century. It was reconstructed in the 14th century, during the reigns of King D. Dinis and D. Fernando.
But, following its alignment with the Kingdom of Castile during the 1383–1385 Portuguese interregnum, the site began to fall into disorder. Beginning in the 1476, the area was reconstructed during the reign of King D. John I, resulting in the construction of a barbican and protection over the well, a large 15 metres chamber that belonged to the Castro family.
At the beginning of the 16th century, there was a graphic survey by Duarte de Armas for his Book of Fortresses, designing the castle with two access gates (the oldest opened to the cliffs) and protected by two towers. In total there were four towers, a keep and cistern. The Torre do Pião (lookout tower) was in ruins at that time. Supported by an alcalde, the urban nucleus included the parish of São Miguel and the parish of São Salvador. In the late 16th century, there was work performed during the reign of King D. Manuel I.
In the second half of the 17th century, the structures were adapted to support artillery, with the construction of earthenworks, batteries and canon emplacements, as well as fortification walls for the colony. This followed the 1704 siege by Franco-Spanish forces, under the command of the Duke of Berwick, but later liberated by the Marquess of Minas. By 1758, surveys of the site indicated one of the towers in ruin. Reconstruction of the fortification walls were undertaken by order of William, Count of Schaumburg-Lippe following the Spanish invasion of Portugal. Similar alterations were built in 1801, a slope was established in order to implant canons.
Major Eusébio Furtado was later responsible for the installation military garrison in 1813, resulting in major works on the castle. But, in 1815, there was an explosion of the magazine powder keg, resulting in the partial destruction of the castle.
An 1831 collapse of a cliff resulted in the destruction of an exterior wall and a time when the Chapel of Santa Maria was in a state of ruin.
Architecture
The fortress consists of three walled courtyards: an outer irregular exterior wall that encircles two other lines of walls. These segments, with staircases to access the adarve, lack merlons. The enclosure includes a main door preceded by Casa do Guarda (guardhouse), a rectangular floored compartment, with arched eastern access door. In the interior, there is an archway, surmounted by an orifice for the coat-of-arms, flanked by an armillary sphere. On the right hand side is an inscription that is indecipherable. On the northern elevation are three cross-sectional bombardments, one on the northeastern angle. The main arched gate in the north covered by vaulted ceiling and false door on the south tower.
The enclosure includes four towers attached on the outer side of the walls. Two have rectangular plans and are devoid of spans, in the north; an identical tower on the east; and a square-shaped tower on the south, with access via a flight of stairs perpendicular to the adarve, presenting a straight lintel doorway in the northern elevation. Inside the enclosure is a cistern next to a cliff, presenting two full arches and absence of cover.
The walled enclosure, adjacent to the main entrance, features a rectangular outline and a full arch door inserted in the south section, that includes battery with four canon emplacements, equipped with two ramps. Inserted into the western side is an irregular oblong segment with full arched door and protrusions that integrates a square-shaped tower on the exterior. A geodetic marker is situated over this segment. From this enclosure are rocky outcroppings with grooves and rubble corresponding to the foundations of other towers and missing wall segments.
The Torre do Pião is located outside the walls, in an elevation bordering the castle, overlooking the ruins of the Church of São Miguel. A square plan, only part of the walls corresponding to the first incomplete register are visible.
Chapels
The Chapel of Nossa Senhora do Castelo is located inside the walls, next to the battery, with longitudinal plan and comprising two juxtaposed rectangles. Its main facade (to the west) is delimited by Tuscan pilasters, torn by a hollowed arch portal with protruding stone and flanked by a small quadrangular opening and gable with cornice. The lateral elevations include doors of straight lintels, and a southern window in the body of the main chapel.
Near the chapel are three trapezoidal tombs and a group of anthropomorphic excavations in the isolated rocks. The ruins of the Chapel of São João includes a perfect arch and is isolated in the eastern flank of the castle.
The ruins of the Church of São Miguel with longitudinal plant composed of two juxtaposed rectangles and absence of cover/ceiling. Its main facade is oriented to the west, and includes full arch door with four archivolts, imposing towers and capitals with zoomorphic decoration.
The unique nave temple is situated on a slope accessed by seven steps and conserves a rectangular altar, fragment of baptismal fountain (in the form of a chalice), columnar base and other fragments of granite. A triumphal arch are supported with posts decorated with armillary spheres. The bell tower is situated on a granite cliff, over two/twin arches.
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