The Sydney Harbour Fortifications are a series of military structures built to protect Sydney Harbour from potential naval attacks. Constructed in the late 19th and early 20th centuries, these fortifications include Fort Denison, Bradleys Head, and Middle Head. They were designed to defend against threats, particularly during times of conflict such as the Second World War. Today, many of these sites are preserved as historical landmarks and are open to the public. Visitors can explore the fortifications, learn about their history, and enjoy stunning views of the harbour. The fortifications serve as a reminder of Sydney's strategic military importance in the past.