+150% settlement defense.
See the list of buildings
A Fortress also reduces the chance of a settlement being raided by natives.
Fortresses are massive defensive structures, capable of housing dozens of cannon, hundreds of men, as well as supplies, munitions and barracks. Fortresses were most often built in areas of high conflict between the various colonial powers. For this reason, no area has a higher concentration of fortresses in the New World than the Caribbean. To defend against the assaults of opposing colonies as well as the occasional pirate raid, many major colonial settlements of the Caribbean built enormous defense structures around their harbors in order to protect them against enemy naval incursions. Many of these fortresses were used for hundreds of years after their construction. The Castillo de San Pedro de la Roca in Cuba was used as recently as 1898 to defend Cuba from the assaults of the United States during the Spanish-American War.