Except for some interior stairs and some retrofitted safety and stabilizing additions, the inside of the Leaning Tower of Pisa is smooth marble. This lovely tour goes all the way up to the bells at the top, offering a great view.
Here’s an interesting video on a project to scan the interior in 3D:
https://www.youtube.com/watch?v=3OUXIYmCoL0
• On the Inside the Tower of Pisa (YouTube / fitasjoias)