The courtyard and the many storage rooms, including the soldiers' sleeping quarters,
can be seen.  The Castillo walls were made of coquina (pronounced ko-kee-na), a
locally quarried soft shellrock.  It was easily shaped by artisans and did not become
brittle nor crumble under cannon fire.  Because of its soft properties, the
cannonballs were absorbed by the coquina, similar to a golf ball falling into
sand.  This made the Castillo unpenentrable by the enemy.