The shape of Castillo de San Marcos, the oldest masonry fortress in North America, provided a stout defense for colonial Spanish St. Augustine. The main walls, which are lined with bombproof vaults for storing food and ammunition, form a large hollow square. At each corner large, diamond shaped blocks called bastions project out. Sentries on the bastions could watch all the surfaces of the forts walls with no hidden blind spots. The angular outline of the Castillo's walls also allows a cannon on one part of the fort to protect or cover the other sides it overlooks. There was no way to come into St. Augustine or approcah the Castillo without coming into the line of fire of the fort's cannon. Engineering such as this was used to build many fortresses around the Spanish Main three hundred years ago. Pin measures 1 in diameter.