The district ends in the awesome Arsenale, the world's first factory. While Venice made its riches through trade, it also developed world class manufacturing capabilities in certain areas, most famously shipbuilding here at Arsenale, and glassmaking on the nearby island of Murano. This enormous complex employed 20,000 workers at its peak in the fourteenth and fifteenth centuries, and continued building ships until World War I, ending a 900 year production run. It was the first assembly line, ingeniously arranged so that the boat being built would be dragged along a canal as workers added various things to it, and continue being pulled through to the last station, where food and supplier were loaded and the crew jumped in. The factory built one ship a day, which is really amazing. As you can see from the photos, it was extremely important strategically, and hence heavily guarded. Unfortunately the site itself is closed until the city can figure out what to do with the huge place, and you can only see part of the massive walls guarding the inactivity inside.