The Tower of Montlhéry

The castle of Montlhéry, first mentioned in the early 11th century, is a landmark in French history. Situated on a hill, it was built by Thibaud, Lord of Montlhéry, who was King Robert II’s guardian of the waters and forests. In the early 12th century, the lords of Montlhéry opposed King Louis VI, and this first castle was destroyed, with the exception of its tower. The estate became part of the crown in 1118. The strategic position of the site, between Paris and Orléans, made it a key issue for the Capetian kings.

Around 1200, under the reign of Philip Augustus, the castle was rebuilt and took on its current form. It adopted a regular layout, with imposing curtain walls, circular towers at each corner and a massive keep. These features are characteristic of the revival of castral architecture under the reign of Philip Augustus. The future king Louis IX, known as Saint Louis, escorted by the Parisians, took refuge here during a revolt by the barons. In thanks, on his return from a crusade in 1254, he had the chapel that bears his name built at the entrance to the castle. Its foundations can still be seen on the left, before the drawbridge.

Later, during the Hundred Years’ War, the castle was besieged and occupied several times by the English. Olivier de Clisson, the French constable in charge of guarding it, therefore undertook major works in 1382. He added 4 storeys to the keep, a sentry walk with machicolations, latrines and chimneys. At the time, the building was one of the most beautiful castles in the Paris region.

Unfortunately, it later experienced some troubled times. It was first damaged during the war between Armagnacs and Burgundians in the 15th century. In the 16th century, its deterioration continued during the Wars of Religion, when it was used as Calvinist headquarters. In 1591, the town’s inhabitants asked Henri IV to dismantle it.

Nevertheless, during the 18th and 19th centuries, the castle found a new use thanks to scientific research. In 1822, a team of scientists including François Arago, astronomer and physicist, improved the measurement of the speed of sound in the air by conducting experiments between the castle tower and Villejuif. The experiment consisted of firing a cannon between the two locations, 18 kilometres apart, and using extremely accurate clocks to measure the elapsed time. Later, in 1874, the physicist Marie-Alfred Cornu calculated the speed of light between the castle tower and the Paris observatory by sending a beam of light along the route. He arrived at a measurement of 300,400 kilometres per second. In the meantime, the castle was included on the first list of historic monuments in 1840 and became the property of the State in 1842.

During the 20th century, the tower underwent various repairs and was temporarily closed to the public in the 1980s. After extensive safety work in the 2000s, it reopened to visitors on 15 September 2012 on a regular basis.