Petra was first established sometime around the 6th century BC, by the Nabataean Arabs, a nomadic tribe who settled in the area and laid the foundations of a commercial empire that extended into Syria. Despite successive attempts by the Seleucid king Antigonus, the Roman emperor Pompey and Herod the Great to bring Petra under the control of their respective empires, Petra remained largely in Nabataean hands until around 100AD, when the Romans took over. It was still inhabited during the Byzantine period, when the former Roman empire moved its focus east to Constantinople, but declined in importance thereafter. The Crusaders constructed a fort there in the 12th century, but soon withdrew, leaving Petra to the local people until the early 19th century, when it was visited by the Swiss explorer Johann Ludwig Burckhardt.