Boulogne was a coastal city in France.
History
In 1805, during the Napoleonic Wars, Napoléon Bonaparte gathered his armies in Boulogne in preparation for an invasion of England. The Players organised anachronistic torpedos for testing at Boulogne to use against the British Navy, but the Second Doctor sabotaged it. (PROSE: World Game [+]Terrance Dicks, BBC Past Doctor Adventures (BBC Books, 2005).)
During the Nazi Blitzkrieg of May 1940 at the start of World War II, Boulogne was captured by the German forces under General Heinz Guderian. The Allied forces, mainly of the British Expeditionary Force, were pushed back to nearby Calais. (PROSE: Timewyrm: Exodus [+]Terrance Dicks, Virgin New Adventures (Virgin Books, 1991).)
In May 1983, Harry Sullivan travelled on a ferry from Dover to Boulogne. (PROSE: Harry Sullivan's War [+]Ian Marter, The Companions of Doctor Who (Target Books, 1986).)