The Tangus river is the longest river on the Iberian Peninsula. It marks the border between the regions of Al-Andalus and Spain. After 1212 (609 AH), it marked the boundary between the Muslim and Christian polities of the south and north.
For the Portuguese the importance of the Tagus is defensive because it provides a natural defense against direct Moor attacks from the south and marks its historical boundary with the south at the maximum expansion of the Reconquista.
For Castilians and Moors it has been a territory in a permanent tug of war, notably during the 13th and 14th century. Both shores of the river had been occupied and regained more then once. Up to now it is still possible to see the lines of fortress (Alcazars) on both shores.