Château de Montaiguillon, Medieval castle ruins in Louan-Villegruis-Fontaine, France.

Château de Montaiguillon is a ruined fortress begun in the late 12th century on a sandy hill in Louan-Villegruis-Fontaine. Its light grey block masonry survives as stretches of wall with uneven, mossy tops, a large rounded tower with a small dark opening near its base, and a second rounded tower beyond a gap in the wall.

Built at the end of the 12th century by the Knights of Saint John of Jerusalem, the fortress guarded the Gallo-Roman "Perré" road linking Troyes to Senlis via Meaux, 1.5 km to the west. Its architects also worked on the royal Louvre, hence similar architecture. In 1613 Louis XIII compensated M. de Villemonté and had it destroyed with explosives; the tower on its sand hill tilted but did not break. Listed as a monument historique in 1875.

Local residents keep the ruin at a distance, considering it "haunted" since the murder of its previous caretaker in 2002; the killer's identity remains unknown. A legend also holds that underground passages once linked the castle to Provins.

The castle is privately owned and visitors need advance permission before coming. Contact the owners beforehand to arrange access and confirm which areas can be viewed, as some sections may have restrictions.

In 1613 Louis XIII paid off the owner and ordered the fortress destroyed with explosives. The charges failed to topple it: built on a 20–30 m sandy hill of Fontainebleau sand, the castle absorbed the blast, and one tower still leans without having broken.