During the siege of 1870 by the German armies, the lines of trenches forming a perimeter of 83km. The new forts have a circle of 126km. The attacker must assemble an army of 700,000 men, which seems impossible or disproportionate to the importance of Paris.
 
The site is organized into three geographic groups:

    
* The first in Northern Cormeilles Stains forces the enemy to cross the Seine to the west of the Oise and can control the plains of France.
    
* The second in the East Vaujours in Villeneuve-Saint-Georges holds the trays where the Prussians bombarded the forts and blocked all exits in 1870.
    
* The Third in Southwest Palaiseau at Marly-le-Roi facilitates exit of an army relief from the Loire or the Atlantic ports.
Between these groups, they remain three holes:

    * The North West is barred by the meanders of the Seine and the forest of Saint Germain
    * Those of the North East and South are traversed by many réivières making the marshland and not likely to attack.

A railroad called "big belt" is a tour of Paris, passing just behind the forts he refuels. It connects camps Satory and Vincennes Railway province facilitating the movement of troops. It is being phased into operation between 1877 and 1883 say 8gares military because of their elongated platform for landing troops rapidly.
Between 1874 and 1881, 18 forts, redoubts 5 and 38 batteries are built around Paris.
 
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