Isaac R.
Google
There is an exhibit hallway up in the building, you can visit it on your way up to the top of the bell tower.
Great view from the top but be careful not to be up there when the bell rings.
If you have the Volterra city pass you can visit the town hall, the art museum, the Roman theater, and the Acropoli Etrusca/Roman cistern up in the park with the same ticket.