[Illustration: This bird’s-eye view of Castillo de San Marcos shows
how it is laid out and why. The fort was located at the north end of
Saint Augustine and on the water for defensive reasons. The moat
protected it on four sides, and the Matanzas River lent additional
protection as well. The only entrance was at the point closest to
the town, so the inhabitants could quickly go to the fort if danger
threatened. The fort was designed, too, so that every wall could be
seen from some vantage point inside the Castillo. No attacking force
could sneak up to the very walls without the defenders seeing them.
The original Castillo was simply the exterior walls. Parallel to
them were the inner, or courtyard, walls, built also of stone. Beams
spanned the space between exterior and inner walls and held up
platforms upon which guns sat aimed at the surrounding countryside
or out over the water. Such a structure offered scant bombproof
defense against incoming projectiles. And the wooden beams were
subject to rot in the humid, subtropical air.