The Porte de Cosne was formerly known as the Porte du Vieux Château de Cosne. In the 9th century the village was surrounded by a wooded fortification until in the 12th century stone fortifications began to be built. Looking closely, one can see that the roof of the gate is exceptionally steep. The tower that originally existed was to be over 20 meters high and closed with heavy gates and a portcullis to prevent enemy attacks.