Map of the Louvre castle under Charles V (reigned 1364-1380) as reconstructed in 1856. The map is inverted with the South (and Seine river) at the top of the image.
The Louvre in 1380 (under Charles V) as imagined in 1885.
Plan from around 1880 of the Louvre castle in 1380 after the construction of the Charles V wall.
Before his departure for the Third Crusade in 1190, King Philip II wanted to protect Paris, the capital of his Kingdom, against invasions. In particular, he wanted to defend Paris against English soldiers based in Normandy. Additionally and more specifically, King Phillip II wanted to build a place to keep his treasure and his archives safe. These archives were especially sacred to the King because they were lost during the Battle of Fréteval in 1194 against Richard the Lionheart, but he had since reconstructed and didn't want to lose again.
He built the enclosure, which bears his name, around Paris between 1190 and 1209 for the right bank portion and between 1200 and 1215 for the left bank which was less exposed. The Louvre castle stands west, the most exposed side, as English soldiers were occupying Normandy less than 100 km away. This decision was also motivated by a previous Norman invasion in 845 that almost captured the city.
The castle itself is composed of a squared fortress (78 m by 72 m), surrounded by a 10 m wide moat fed by the water of the nearby Seine river. The wall on the west side, which overlooks the countryside, is thicker and doorless when compared to the other walls, as it was appraised to be the most exposed side, and therefor the most vulnerable for an attack. The perimeter is reinforced by ten defensive towers, with extra emphasis on the corners of the castle. The space between towers never exceeds 25 m, which is the distance corresponding to the effective range of a bow (of that time). They are pierced with arrowslits to defend the ramparts. The castle has two entrances with the main one facing south and the Seine, while a secondary smaller one faces east and the city. These doors are protected by drawbridges and are framed between two twin towers. Two additional buildings housing the garrisons and the arsenals are located outside of the surrounding wall, to the west and south of the central courtyard, respectively.
The set of curtains and walls are at a slight downward angle to hamper sapping attempts and to facilitate the rebound of projectiles thrown from the top against attacking soldiers.
A dungeon named the Grosse Tour (Big Tower in french) was built in 1200AD in the center of the courtyard. It is a circular dungeon with a diameter of 15.6 m and is enclosed by a 30 m tall wall that is 4.25 m thick at its base. It is surrounded by a 9 m wide ditch that is 6 m deep. This ditch is dry (i.e. not flooded) and paved with large irregular stones. It is crossed by a drawbridge. The interior arch was built in stone to limit the risk of fire. The dungeon has a conical roof slate over the machicolation. It features a well and a large tank for supporting long sieges, as well as a chapel.
Philippe Auguste chose a round dungeon, instead of a squared or rectangular one, for military reasons. The reason being, enemy pioneers could more easily sap the wall at the angles of squared towers compared to circular towers, a smart tactic on the part of Auguste.
The dungeon initially had a military function as the refuge of the king. However, it essentially housed the royal treasure and archives, at least until Philippe le Bel. It was also used as a prison until the 14th century. Notable prisoners included Ferdinand, Count of Flanders, who was defeated and captured at the Battle of Bouvines in 1214 and then spent thirteen years imprisoned in this dungeon.
This castle is a typical example of the philipian architecture. (fr)