Marvão castle was a strategic detention fortification, oriented towards the border, which is a mere 13 km away. It also constituted an effective place of refuge and an extraordinary point of observation and surveillance, since it clearly dominated the second most important route of penetration of the armies of the neighboring country, from Valencia de Alcântara, in a vast area of ​​the Alto Alentejo that goes from Badajoz to the Tagus river. Its strategic insertion is clear: it is part of the first detention line, after the Treaty of Alcanizes, which runs, in the current District of Portalegre, from Montalvão to Elvas. The most rocky of the national castles, it is constituted in two contiguous enclosures that, for defensive reasons, had to embrace all the highest rocky crest.

Regarding the inaccessibility of the fortification, the following description by Prior Frei Miguel Viegas Bravo, in the Parish Memories of 1758, is interesting: It is this square of arms, the most unconquerable of the whole Kingdom; from the south it is inaccessible, in such a way that only the birds allow entry, because along the entire length it is continuous, and the cliff of living plumes continues at such a height, that the birds of higher flights, let it be seen from behind (...) which wall, serves more to not let those inside fall, than to prevent the entrance to those outside, and therefore in many parts it is this low wall (...) that this square or prison cannot to have against you in war time more than the lack of water. The urban fence occupies the rest of the rocky crest. The entire fortification is obviously in relief, partly natural, with a clear concern for impermeability.