The Secret Harbour is located at the southern end of Wild Cat Island. It is a sheltered inlet, protected from view by high rocks and sheltered from the weather. The safe passage into the harbour is straight but narrow and hazardous. There is a string of rocks extending out into the lake which also make it more difficult to see and access.
The harbour was discovered by the Swallows during their initial exploration of Wild Cat Island, but they were only able to negotiate the entrance by careful trial and error. The Amazons were more knowledgeable about the harbour and had painted a cross on a stump which gave them leading marks when combined with a distinctive fork in a tree. This allowed them to steer straight down the channel without having to worry about the rocks.
During the war between the Swallows and Amazons in Swallows and Amazons, John Walker realised that by adding lights to the secret marks, Swallow could use the harbour at night. However, it was the Amazons who eventually made use of the leading lights.