Inside Fort Macon's southwest corner. Each of the fort's three rounded corners were built to be the most secure, notionally bombproof parts of the structure: These were where the powder was kept. This is the outside wall of the powder magazine.

I kind of assumed that this wall was constructed in this manner so as to resist the explosive power of an incoming round, thus protecting the powder magazine, but Fort Macon expert Paul Branch assures me that the wall is this shape because it was built to conform to the curve of the fort's wall. And Paul Branch certainly seems to be the authority, so I guess I believe him.