The fort that "swallowed" cannonballs

Message:

The composition of the walls of Castillo de San Marcos in Florida is near-unique (only two fortresses in the world have this composition), and would be a great thing for Red to adapt to a "New World" fortress in Cutlass.

So here's the Atlas Obscura article linky: https://www.atlasobscura.com/articles/coquina-fort-in-florida.amp

Yours,