On 13 May advance elements of the German 71st Infantry Division approached La Ferté (which the Germans called "Panzerwerk 505") and occupied the surrounding area, out of range of fire from the "ouvrage". The 71st ID had trained specifically for an assault on the Maginot Line, and were provided with shaped charge explosives of the type used in the assault on the Belgian Fort Eben-Emael a few days before. The night of the 13th and 14th La Ferté installed a periscope in its machine gun turret, which had just been delivered on the 11th. On 15 May the Villy Est casemate opened fire on German troops advancing on a nearby farm. On the 16th the Germans took nearby Hill 226, which overlooked La Ferté from the west and attacked the higher Hill 311 (overlooking La Ferté from the southwest) despite fire from the Villy Ouest casemate. That evening, German reconnaissance around La Ferté and the Moiry and Sainte Marie casemates drew supporting fire from Ouvrage Chesnois's 75mm guns. Chesnois fired 1200 rounds in seven hours. On 17 May, German forces made a series of determined attacks on Hill 311, and German artillery began bombardment of La Ferté with 21 cm mortars and 88mm high-velocity anti-tank guns. About midday on the 17th, La Ferté's telephone communications were cut, forcing the position to communicate by radio, which could be monitored by the Germans, and which required Bourguignon to transmit through Chesnois to reach headquarters.