The ramparts, of which only scattered ruins remain, once rose from the crest of a steep hill above a stream that flows into the Yvette.
This was once a place of some importance, chosen in the 10th century or 11th century as the capital of a district under the diocese of Paris. Châteaufort in those days had two churches and a priory: one church for the town, adjoining the priory, and the other for peasants living without the walls, in the hamlet of La Trinité. The first still stands; the second has disappeared.
In the 11th siècle, Gui de Montlhéry was lord of Châteaufort and his son, Hugues de Montlhéry, Grand Seneschal of France, inherited the title in 1112.
In the 13th century, there was a leper colony here1.