Why did Spanish authorities build forts around Havana Harbour?

Spanish authorities fortified Havana Harbour primarily to protect the city and the valuable Spanish treasure fleets from foreign threats. During the 16th and 17th centuries, Havana served as a vital rendezvous point where ships carrying gold, silver, and other colonial goods from the New World gathered before crossing the Atlantic to Spain.123

Because these fleets carried immense wealth, they became prime targets for English, French, and Dutch sea marauders. The natural deepwater port was recognized as the "Key to the New World," making its defense a strategic priority for the Spanish Empire. By constructing extensive fortifications, the Spanish aimed to secure this staging area, which was essential for maintaining their economic and political dominance in the hemisphere. These defenses were so significant that they allowed the city to withstand numerous attacks from competing foreign powers throughout the colonial era.4251