Focusing on the example of Sintra during the Islamic period, we seek to understand how the knowing of the territory weighed in the strategy for the construction of fortifications and surveillance structures that allowed for the military defence and that were part of the defensive system of the Lisbon region. The methodology used consists on the crossing of data between historical and archaeological sources, toponymical and geographical recognition. The results show that Sintra, besides two castles and a ribat, will have had other defensive structures and checkpoints that allowed communication with other distant places and marked the maritime cultural landscape. This investigation allows to conclude that the structuring of this defensive system implied a strategic planning based on the study of the different geographical combinations, the relations between the villages, the communication ways, the ports and anchorages, and that some important places would already have been used in previous eras.