Summary: flat is fine, but forget about sleeping. The windows and shutters are old and definitely not sound proof. Some bars play loud music till 2am on the weekend, and the flat is in one of the busiest restaurants area of the historical center. If you're planning to party it's fine (though expect the bin truck to wake you up at 6am). If you wanted to sleep before midnight, go somewhere else, the street party is as loud as if it was in the bedroom.
The company is not responsible for the street noise, and it's normal that touristy places are noisy... But the rental company could fix the windows and change them for modern ones, to ensure comfort for the guests, which they have not.
The rest of the flat is fine, and it's well located to visit the historical center. But we ended up cancelling and go further away. Good cancellation policy. In short: only recommend for party people.