Since the second half of the 11th century, the kings of Aragón and the counts of Barcelona and of Urgel tried with obstinacy to conquer the Muslim held towns and frontier fortresses of the Marca Superior. Specifically, they targeted the low lands around the Segre and Cinca Rivers all the way to the mouth of the Ebro, an active and prosperous region with direct access to the Mediterranean Sea. The most important towns in this region were Lleida, Mequinenza, Fraga, and Tortosa. The region was rich in commerce and had a very productive agricultural system. It also was a region that was constantly plagued by warfare throughout this period. For this reason, the land is dotted with many fortresses, towers and even subterranean refuges where people would hide from attack.