These water mills played therefore a very important role in the life of the village. They not only produced flour, which used to be the base for food then but these mills also used water as an energy force which at a time when human labour and animal traction were most usual, water produced energy was seen as heavy industry.

In this respect, only powerful and wealthy people could afford to own them. The lords set up this true public service for which operators were paying duties called “banalities” amounting to the running title and the maintenance cost.

The system dates back the medieval times and for the most of it well before the beginning of the XVth century when mentioned for the first time in available documents.

The most ancient of these mills existing today is mentioned in 1433; two of these mills existed at that time already. The other two were built between the beginning of the XVth century and the middle of the XVIIth century.

In 1665, the cadastral register of Castelnau certifies the presence of four flour mills with their houses and ponds , the upper one being the best.

During the XVIIIth century, a part of the open sky aqueduct was covered with pavement by the “Griffoul” street. One of the mills disappeared during the XIXth century . The three left have been in operation long after the second world war. They cease operating soon after 1960.

The waterway which has been going through the village for centuries for the best interest of all the residents went through many alterations. The complete water supply was finally abandoned. People start now to recognise its value and the need to restore it.

In order to transport water on to the rock where Castelnau sits, artisans had to build an aqueduct, for a part open to the sky and partly underground.

From thereon the aqueduct goes underground through the external surrounding wall to reach the base of the village walls. Then it goes sometimes open to the sky, sometimes as a stone cut conduit through a ravine so called “ruisseau des moulins” in reference to the mills staggered alongside its steep slope.

One can see a number of elements which are very ancient too all the way along the course of the aqueduct which first function was likely to supply water to the castle.

These reservoirs were used to collect and regulate the flow. The most important , called “l’étang” which means “the pond” is situated above the village. This pond was designed to collect water running from the plateau and worked as a flow regulator for the entire water supply system.

A second dam, far modest in size, was located at the level of the second mill. Lower mills were likely to have one too.

It was designed to supply the castle with water. However by its natural upstream position, it was giving the necessary pressure to allow Castelnau’s mills to operate.

Several overflows were going from the aqueduct to irrigate the fields around the village. With the time, agreements were concluded amongst the various landowners to rule the days and hours within the week during which water would run through the different overflows.