Last weekend when we were geocaching around Ribemont, we encountered the remnants of the abbey of Saint-Nicolas-aux-Bois. It used to be a Benedictine abbey, building was started around 1024. Today, parts of the buildings date from the 15th century, including the great tower with the two turrets and the (now caved in) main hall (source).
The abbey around 1660.
The abbey around1869.
Today the abbey looks rather different, the main hall has caved in and so has the chapel. Most of the exterior buildings are still in good shape and inhabited. I just think t is a great shame that the main building has not been restored, or kept in the first place.