The fortifications of Paris in the 19th and 20th centuries comprise: * The Thiers Wall, surrounding the city of Paris, and farther from the city, * The detached forts and their complementary fieldworks. They were built in two stages: * 1840-1845: the Thiers Wall and the first ring of forts, about five kilometers from the city; * 1874-1885: the second ring of forts, about twenty kilometers from the city.