Most city property such as city hall, city parks and other city owned property is considered to be public property.
The tax payers who live in that city or county pay for that property and you can be legally on the property as city hall would be considered public property and considered to be the peoples building.
A mayor of a town told me that before and said that I was welcome to walk around any part of the city hall and said that it's everyone's building.
This was a city hall when I was traveling out of state and I had an interest in the building and he said I was welcome to video in the city hall and take pictures and I wasn't even living in that town.
Now there might be areas such as a city shop etc where you should not be going on the property or at least not going on there after hours.
But other places of city property are okay.