These parishes predate the sestieri, which were created in about 1170. Each parish exhibited unique characteristics but also belonged to an integrated network. Each community chose its own patron saint, staged its own festivals, congregated around its own market centre, constructed its own bell towers, and developed its own customs. Other islands of the Venetian Lagoon do not form part of any of the sestieri, having historically enjoyed a considerable degree of autonomy. Each sestiere has its own house numbering system. Each house has a unique number in the district, from one to several thousand, generally numbered from one corner of the area to another, but not usually in a readily understandable manner. Economy 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 centre 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. Armenian merchants from Julfa were the leading traders in Venice, especially the Sceriman family in the 17th century. They were specialized in the gems and diamonds business. The trade volume reached millions of tons, which was exceptional for 17th century. This all changed by the 17th century, when Venice's trade empire was taken over by countries such as Portugal, and its importance as a naval power was reduced. In the 18th century, it became a major agricultural and industrial exporter. The 18th century's biggest industrial complex was the Venice Arsenal, and the Italian Army still uses it today (even though some space has been used for major theatrical and cultural productions, and as spaces for art). Since World War II, many Venetians have moved to the neighboring cities of