Zuko and Katara infiltrated one of these towers in an attempt to locate the Southern Raiders, whose leader murdered Katara's mother, Kya, six years before. They found plans for the unit's patrols, and eventually tracked their flagship to Whaletail Island. This particular tower was isolated on a large, bulbous island of rock and its base was comprised of several facilities.
Fire Nation Navy engineers erected dozens of these giant towers throughout the ocean following the start of the Hundred Year War. These tall towers send messages to the various ships and fleets sailing across the globe, and keeps the navy informed on battle plans or enemy attacks. This is done entirely by messenger hawks; the towers provide housing for the hawks and landing areas where their messages can be checked by intelligence operatives that work in the towers. The towers also serve as command centers where commanders assemble to create the plans they send out. Every tower has to be up to date on where every ship in the fleet is deployed, making them a prime target for counter-intelligence operations.