The city's development followed that of Portuguese nationality. As the Reconquest advances to the Mondego line - the conquest of the city of Coimbra (1064) being an important milestone - it became important to reinforce the military defense of the new Kingdom, not only in the South, where the Almohads tried to recover territory , but also to the East, where the Kingdom of Lion sought to add territory. Repopulation was an important part of this process of strengthening, not only the borders but also the natural gateways, with small communities being installed with fortifications in strategic locations. Due to its importance in terms of military strategy, D. Sancho I granted a charter to the Guard in 1199, to enable it to become an administrative and commercial center. It is thought that before that, the Guard would be just a small community guarded by a small watchtower or tower (also called “guard”). The construction of the castle, by King Povoador, in the first quarter of the 13th century, contributed to its growing grandeur and strategic importance.

Shortly after the city was founded, the old cathedral of Egitânia (now Idanha-a-Velha), founded in the mid-sixth century and abandoned after the Muslim invasion of the eighth century, was transferred to it. It was its first bishop, D. Martinho, in 1203. The dignity of an episcopal city was granted at the request of D. Sancho I by Pope Innocent III.