A gun emplacement was built in 1801 on Middle Head (north of Obelisk Bay) during the Napoleonic wars, as well as another one at Georges Head. Fort Phillip was also built to protect from a potential French invasion in 1804, as well as following the Castle Hill convict rebellion the same year. More permanent fortifications were built during the first half of the 19th century in response to feared foreign attacks with new threats to the British empire, including Napoleon's French troops in 1810, and the Russian Pacific Fleet in the 1850s (during the Crimean War). In the meantime two American warships (sloop of war "Peacock" and sloop of war "Vincennes") entered the harbour unannounced on 30 November 1839, under the cover of darkness. Although only a courtesy visit, the event demonstrated the settlement's vulnerability to attack and highlighted awareness of its isolation and wealth. Garden Island had fortifications built in the 1820s, while Bradleys Head started building in 1840. Construction of Fort Denison began in 1841, though the fort was not completed until 1857.