The nefarious Marquis Montalban is the main antagonist in "Sid Meier's Pirates" for the PC. Although that is not saying much, as he never actively pursues your protagonist and only appears when you go looking for him in specific towns. He is the evil Spaniard who has captured various members of your family. His fortress is hidden somewhere in the Caribbean in a random location for each game.
Defeat the Marquis Montalban several times in one-on-one duels. Governors and rumormongers may be able to tell you where Montalban is hiding out if you do enough favours for them.
Collect map pieces from Montalban after every defeat. The map pieces will combine to show you the way to his hidden fortress.
Sail to the location on the map and disembark to walk the lands on foot.
Follow the map markers -- geysers, totem poles and Indian ruins -- until they lead you right to the fortress.