The Castillo design evolved from the medieval castle. A towering castle was too vulnerable to artillerist. So they lowered their target-like walls, and added large mounds of dirt to stop the cannonballs before they could hit the stone. And to keep invaders from scaling, they built large moats all around.
Today, when opened, you can tour the interior courtyard as well as visit the gun deck which offers a commanding view of the city and bay. Cannon firings and gun demonstrations are conducted most weekends, but sadly with the coronavirus, the fort is closed.
And on this balmy spring evening, it protected me, as I slept beside it, in the Nation’s oldest city