As early as 600 BC the Iberians built a fortified town on a rocky hilltop perched over the valley of the Aude. Their settlement included huts with walls made of clay mixed with stones and straw, and with tiled roofs. But it was the Romans who built the stonewalls in the late 3rd century and gave Carcassonne the appearance it still has today. A 1,200 m long defensive wall at the crossroads of ancient trading routes linking the Atlantic with the Mediterranean and the Iberian Peninsula with the rest of Europe. After the Roman Empire had fallen, the fort was conquered and re-conquered by many different rulers: the Visigoths, the Arabic Saracens, the Franks. Things only really started to calm down under the rule of the Trancavels between the 10th and the 13th century. The key to the wealth of the Trencavels was salt - through which they not only built an empire but also a unique habitat for seabirds.