Final answer:
Several factors contributed to the longevity of Venice structures and systems of rule: a stable republican government, wealth from trade, and a strategic location.
Step-by-step explanation:
The longevity of Venice structures and systems of rule can be attributed to several factors. Firstly, Venice possessed a stable republican government led by a Doge, which provided continuity and effective governance. Secondly, the city benefited from its wealth derived from trade, particularly with the Levant, which allowed for the maintenance and improvement of its structures. Lastly, Venice's unique location as a gateway between Europe and Byzantium gave it strategic importance and contributed to its longevity.