After the fall of Rome, Venice was the doomsday bunker of the elites. Extremely defendable position (back when cannons didn't exist), close enough to the shore to transport things and people quickly, yet far enough that it's impossible to besiege. They pooled all their money to build a city on water while everything else around them was on fire both politically and literally.
in the context of the caliphate's eating of Roman territory the idea Constantinople was unsafe actually held a lot of water, with Constans II moving the capital to Syracuse and 10 years after his death the arabs very seriously threatened it in the famous 4 year siege