Genoa also plays a large part in the crusades, establishing strong trading links in the eastern Mediterranean and coming into direct competition with Venice.
Warfare between these two Italian city states is long and intermittent, with Venice by no means always the stronger - until the issue is finally resolved in 1380 at Chioggia.
The Normans drive the Byzantines from their last seaport in southern Italy by 1071, and soon they begin raiding across the Adriatic; in 1082 they take the important harbour of Durrës (or Durazzo) in Albania.
In return for help against these marauders, the Byzantine emperor grants Venice an astonishing concession.
The doge, richly attired and publicly honoured, is a powerless figurehead at the centre of this state administration.
The system is brilliantly devised to preserve the status quo in two ways - preventing the present doge's family from acquiring power and preventing the wider group of patrician families from losing it.
The other new development of the late 11th century opens up extra routes for trade, along the coasts of the Byzantine empire and beyond.In 1096 the contingents of the First Crusade set off eastwards to recover the holy places of Christendom from the Muslims. By the end of 1099 there is a Latin kingdom of Jerusalem.A great increase in trade, travel and pilgrimage to the eastern Mediterranean is the inevitable result.Their labours produce probably the most sumptuous church interior in the world, with every corner a sombre glittering gold.It has been calculated that the mosaics of St Mark's cover an area of about an acre.Venice's long link with Constantinople is evident in the mosaics, in the Byzantine style, for which the islands of the lagoon are famous.