In 1211, on July 30, D.Afonso made a letter of donation and perpetual security in Coimbra, to the master of Évora D.Fernão Eanes, of a land located between Santarém and Coruche. Due to its good natural characteristics, the strategic importance of its location soon became apparent. It is believed that there is a Castro Lusitaniano, belonging to the first Iron Age. In fact, there is no doubt as to the reason behind the choice of the location for the establishment of the primitive medieval fortress of Avis. The headquarters of the future Military Military Order of Aviz, transferred there by Afonso II of Portugal, around the year 1211, resulted from the institution, on August 13, 1162, by Afonso I of Portugal. Its knights were at that time dependent on the Order of Calatrava, from which they only freed themselves in the time of John I of Portugal.

D.When donating the land, Afonso II made it clear that he wanted to see a castle built on top of a hill, as well as the obligation to build the town and the convent church, the future headquarters of the Order.

The fortification and construction work on the castle took place around 1214, when the Order was headed by Master D.Fernão Anes. The first phase of completion of the works ended around 1223, at the time of the master's degree of D.Fernão Rodrigues Monteiro, with a second charter being granted to the town on August 20, by Martim Fernandes. This charter, as it was granted by the master of the Order, was not registered in the Chancelleries, so there was no royal confirmation or registration of the said charter of 1223.

The territory granted to the Military Order of Aviz at this first moment was distributed over a vast area and was located in the area between the Tagus and the Guadiana, from the line that passed through Santarém, Coruche, Abrantes, Elvas. The master's degree came to have 21 villages, 49 commanderies, 71 priories and to appoint the priests of 140 parishes.

Legend has it that the Master and his Knights, looking for a place to settle near the lands bordering the Moors, after leaving the village of Vaiamonte, saw two waters above a tree, taking this as a good prognosis, decided to begin, on the 15th of August, to settle the place, where the village of Aviz came to be born, in consideration of the said Aves; These would be forever remembered in the town's coat of arms, to which is added the cross with fleur-de-lis on a gold field, with fleur-de-lis finishes in the form of the Military Order of Calatrava. Baptista Lima states that the word Aviz refers, above all, to the abundance of birds in the area.

The castle was built with a walled enclosure, surrounded by 6 towers, of which the Torre de Menagem stood out. The existing gates were: Porta D´Évora, Porta de S. Roque, Porta Debaixo and Porta do Postigo. Of the six towers that existed in the Medieval period, only three remain perfectly visible: the Queen's or Convent's, S. Roque's and Santo Antônio's. The Keep was located next to the Old Palace of the Prior Major, where the services of the Municipal Council of Avis are today. It was built by Constable D.Pedro, in the 15th century, who was the mentor of the main works on the Conventual building during this period and who left for posterity a magnificent cistern, with a star-shaped mouth, in the old cloisters of the Convent, with its motto Paine pour Joie (Pain for Glory), recalling the nobility of spirit that accompanied his personality.

Of the original convent building, only the medieval foundations remain, which have since been occupied by successive constructions that have added to and modified the space in its appearance and functionality; From the Manueline campaigns we have the testimony of the Cloister, the magnificent Chapter Room, currently occupied by the Municipal Museum, the Monks' Room, and also the Sacristy of the Convent Church, where the mark of the illustrious patron. Jorge de Lencastre, the illegitimate son of John II of Portugal, would remain, to this day, as a witness to his Government at the head of the Military Order of S. Bento de Avis.

The town's toponymy has crystallized to this day terms that denote activities to which its residents dedicated themselves, such as Rua dos Mercadores, for example, Travessa do Forno, or the presence of ethnic minorities, such as Rua da Mouraria.

The growth of the Medieval Town followed an urban structure typical of the Gothic Quarter, with two perfectly distinct nuclei being visible: on the one hand, the area where the old headquarters of the Convent of S. Bento de Avis is located, to the east of the town, and to the west, another reality consisting of public spaces and spaces for the exercise of civility, such as the pillory, the Medieval Town Hall, the squares and spaces of worship, the main church, the procession steps, and the Chapel of Misericórdia.

The Conventual Nucleus consists of the Reading Cloister, the Monks' Room, the Chapter Room, and the Sacristy, all of which date back to the Manueline period, although in the case of the Sacristy, the decorative elements are from a later date. The Church, founded in medieval times, now has a modern appearance, as does the southeastern part of the Convent known as the ruins of the New Convent, in which the hand of the royal architect Baltazar Alvares is clearly evident. In decorative terms, the church's interior dates back to the Baroque works, which reached their peak with the creation of the magnificent altarpiece (1694) in the National style, the work of Lisbon master José Antunes, which stands out for never having been gilded. Also noteworthy are three high-quality Baroque sculptures, a Our Lady of the Assumption, a Saint Benedict and a Saint Scholastica. It is also worth mentioning the standard tiles that cover some of the chapels of the old monastery, as well as the magnificent high choir from the period of D.Maria I, to which the vast majority of the decorative elements of the windows, altarpieces and side chapels also belong.

With the arrival of the Modern Period and the loss of influence of the Military Orders on the national scene, there was also a decline in the influence and power of the Military Order of St. Benedict of Avis, now removed from the golden age of the Medieval Period.