Your question: Florence is built on both sides of the Arno River.
or
Florence was built on both sides of the Arno River.
You are asking which sentence is correct.The building process of Florence Was finished long time ago. So, you can't use 'is' in the sentence if you want to use past tense. However, 'was built' implies that Florence was built by someone else. Look at the following sentences:
John ordered daisy to go to the market.
Daisy was ordered to go to the market.
The first sentence is active voice statement. The second sentence is passive. In passive voice sentence, we can guess that daisy was ordered by someone else to go to the market. Your Passive voice sentence(Number 2 sentence)also has been used in this sense.
I hope it helps.
