The British returned the islands to France under the terms of Article 3 of the 1802 Treaty of Amiens; the last British forces left the islands in May 1802. Napoleon ordered the islands fortified and the work began in 1803. The primary defences consisted of a fort on Île du Large, 170 meters in diameter. The main structure is a circular fort with a diameter of 53 meters, with 48 firing ports for cannon on two levels of 24 casemates each. This work was completed by 1812. There are seven underground chambers and a cistern. The fort could accommodate 500 troops.