Venice's economy has changed throughout history. Although there is little specific information about the earliest years, it is likely that an important source of the city's prosperity was the trade in slaves, captured in central Europe and sold to North Africa and the Levant. Venice's location at the head of the Adriatic, and directly south of the terminus of the Brenner Pass over the Alps, would have given it a distinct advantage as a middleman in this important trade. In the Middle Ages and the Renaissance, Venice was a major center for commerce and trade, as it controlled a vast sea-empire, and became an extremely wealthy European city and a leader in political and economic affairs. From the 11th century until the 15th century, pilgrimages to the Holy Land were offered in Venice. Other ports such as Genoa, Pisa, Marseille, Ancona, and Dubrovnik were hardly able to compete with the well organized transportation of pilgrims from Venice.