As early as 1598, they dug enough to build a gunpowder storage magazine. But they had neither the manpower, the engineering skills nor the tools to excavate enough for a large structure. Finally, in 1671 large-scale quarrying began in the stone pits. Coquina rock is relatively soft and easy to cut while in the ground and hardens when exposed to air. The Spanish learned to waterproof the stone walls by coating them with plaster and paint.
Coquina rock has a unique feature that made it ideal for a fort.
When besieging ships bombarded the Castillo, the walls simply absorbed the cannon balls. It is covered by sand, except along some stretches of beach in Flagler, Martin and Palm Beach counties. Washington Oaks State Gardens at Palm Coast and Blowing Rocks Preserve in Jupiter are excellent places to see coquina outcroppings. The rock is a mixture of shell fragments and quartz grains bound together by calcium carbonate. It began to form when sea levels were higher and today's coast was underwater.