“The Mystery of Florida’s Cannonball-Eating Spanish Fort”
The rock it’s made from, coquina, is a sedimentary rock made up of compressed seashells. It turns out that under impact, the component shell fragments can shift, absorbing the impact. This explains why cannonballs simply sank into the walls like pressing into foam instead of shattering them.
https://www.atlasobscura.com/articles/coquina-fort-in-florida