Puerto Rico was the Gateway to the Caribbean.  The country that controlled that small island could control all the riches of Mexico, Central and South America.  Everything that Spain sent to their American colonies went to Puerto Rico first.  Everything they sent back, including many fortunes in gold passed through Puerto Rico.  This island had to be defended at all cost so it's most prominent pieces of architecture are gigantic fortresses.  It's hard to get the scope of a fortress when you are inside but even from there they were impressive.  And since they all face the sea it's hard to get a shot from far away.  El Morro Fortress, not the largest but perhaps the most impressive, is also designed to be defended from an infantry invasion from behind.  The beautiful lawn in the included picture was actually a military feature that forced invading troops to cross a long open field as they approached.  It was actually a killing field, belying it's beauty today.