Cartagena's geography funnels the fleet through Bocachica

Cartagena combined difficult surrounding terrain with two southern approaches into its bay. The useful channel at Bocachica was deep but so narrow that ships had to pass one at a time beneath Fort San Luis, while the alternative entrance was unsuitable. The port's silver and commercial traffic also made it rich enough for Vernon to prefer it over the expedition's intended target at Havana. Earlier attacks by Francis Drake and a French privateer explain why Spain repeatedly rebuilt and modernized the defenses around this valuable harbor.