Despite the existing controversies, the truth is that there is no evidence of previous occupation of the place until Christian times. It is probable that in the 12th century there was a town in the valley to the east of the Serra da Penha. The name of Portalegre would come from the fact that one of the important activities of the village would be to give shelter and food to travelers (hence the name of port, crossing point or supply). It is also a peaceful (happy) place, especially for the contrast of its green slopes and valleys with the more arid and monotonous landscape to the south and north.

With this movement the population prospered and it is known that in 1129 it was a town in the municipality of Marvão, becoming the seat of the Council in 1253, receiving foral in 1259 by D. Afonso III, who seems to have ordered the construction of the first fortifications which they were not completed.

It was King D. Dinis who in 1290 ordered the construction of the Portalegre Castle, which denotes the strategic importance it had for the king in defending the Alto Alentejo border with Spain, from which it is only 11 km away. in a straight line. This fact caused a flourishing economic development of the town in these centuries of the Low Middle Ages.

When D. Afonso III died in 1279, the infant D. Afonso tried to succeed him on the throne, claiming that D. Dinis was an illegitimate son. Portalegre was involved in this lawsuit since the Infante Afonso took refuge in it and, in 1299, was surrounded by D. Dinis. The siege lasted five months, and then D. Afonso surrendered. It is curious that D. Dinis surrounded the walls that he himself had built in 1290. After the siege, the king remodeled the citadel and the household tower and built a second one, which still exist today in quite a few sections.

In the dynastic crisis of 1383-1385, with Castilian implications, Portalegre also played a unique role. In fact, the position of the steadfast supporter of Leonor and faced with the Mestre de Avis (future João I), raised the population that surrounded the castle and forced the warden of him, Pedro Alvares Pereira, prior of Crato, to flee. The former mayor would die in 1385 in the battle of Aljubarrota, who fought on the opposite side of his brother Nuño from him. The mother of the Alvares Pereira brothers, Frío Gonçalves, lived at that time in “Corro” (now Plaza de la República). In gratitude to the attitude of the population, João I, granted the town the title of "very loyal"

During the War of Restoration, the defense of this stretch of the border once again acquired strategic importance. For this reason, modernization and reinforcement works of the defensive complex began, which were developed between 1641 and 1646, building a bastioned reinforcement in certain sections of the fence, in an original combination of modern systems and the old medieval wall

Some defenses that were certainly insufficient to prevent the occupation by Spanish-French troops in 1704 during the War of the Spanish Succession and by the Spanish in 1801 during the War of the Oranges.

Instead, during the Peninsular War, Portalegre rebelled against invading Napoleonic troops stationed in the region in 1808 commanded by the French general Loison (alias "the lever").

Between the seventeenth and eighteenth centuries, the prosperity of the town is attested by the numerous civil and religious buildings in Baroque style, constituting a powerful commercial and industrial bourgeoisie.

Portalegre becomes the capital of the district with its name, in the formation of the districts on July 18, 1835.

Urban growth imposed, from the 19th century on, the abandonment, ruin and destruction of various sections of the military complex, classified in the 20th century as a National Monument by decree published on June 29, 1922.

In the 1960s, consolidation, restoration and reconstruction works take place. Several constructions attached to the old walls were eliminated, reconstituting the parapets and battlements in various sections.

In 1999 a Museum on military armory was inaugurated in the castle, which includes pieces from the 15th century to the First World War. Finally, in the mid-2000s there were interventions on some parts of the walls, which became more accessible to the public, in particular a section immediately north of the castle and another section along Carrer de Figueira, below the yellow palace .