One hypothesis is that Caesar set up his camp at the current location of the old City of Boulogne-sur-Mer.
Some historians believe that the Old City was built on the camp; at a vicinity to Itius port which he cites in Chapter IV of its Gallic Wars: The second camp was established by Napoleon Bonaparte in 1803 and continued until 1805.
On May 16, 1803, Britain (without a previous declaration of war) seized a hundred French and Batavian ships.
France then declared war and Napoleon chose Boulogne-sur-Mer as a base for attacks.
The Boulogne camp housed about 60,000 soldiers in 1805, and was divided into two large camps: At the top of the cliff were the command barracks, including that of Napoleon, but the headquarters was located at Castle of Pont-de-Briques.