The city was founded in the pre-Ancient Rome period by the Vacceos, a Celtic tribe, as one of a pair of forts to defend their territory near the Duero river. In the 3rd century BC, Beto laid siege to the city. With the fall of the Carthaginians to the Romans, the city of Helmantica, as it was known, began to take more importance as a commercial hub in the Roman Hispania due to its favorable location. Salamanca lay on a Roman road, known as the Vía de la Plata, which connected it with Emerita Augusta (present day Mérida) to the south and Asturica Augusta (present-day Astorga) to the north. Its Roman bridge dates from the 1st century, and was a part of this road. With the fall of the Roman Empire, the Alans established in Lusitania, and Salamanca was part of this region. Later the city was conquered by the Visigoths and included in their territory. The city was already an episcopal see, and signatures of bishops of Salamanca are found in the Councils of Toledo. Salamanca surrendered to the Moors, led by Musa bin Nusair, in the year 712 AD. For years this area between the south of Duero River and the north of Tormes River, became the main battlefield between the Christian kingdoms and the Muslim Al-Andalus rulers. The constant fighting of the Kingdom of León first, and the Kingdom of Castile and León later against the Caliphate depopulated Salamanca and reduced it to an unimportant settlement. After the battle of Simancas (939) the Christians resettled this area. After the capture of Toledo by Alfonso VI of León and Castile in 1085, the definitive resettlement of the city took place. Raymond of Burgundy, instructed by his father-in-law Alfonso VI of León, led a group of settlers of various origins in 1102. One of the most important moments in Salamanca‘s history was the year 1218, when Alfonso IX of León granted a royal charter to the University of Salamanca, although formal teaching had existed at least since 1130. Soon it became one of the most significant and prestigious academic centres in Europe. During the 16th century the city reached its height of splendor (around 6,500 students and a total population of 24,000). During that period the University of Salamanca hosted the most important intellectuals of the time; these groups of mostly-Dominican scholars were designated the School of Salamanca. The juridical doctrine of the School of Salamanca represented the end of medieval concepts of law, and founded the fundamental body of the ulterior European law and morality concepts, including rights as a corporeal being (right to life), economic rights (right to own property) and spiritual rights (rights to freedom of thought and rights related to intrinsic human dignity).