The northern cistern of the « castle » part of the site is the largest of the three cisterns known so far at Poilvache. It was only partially built underground, which explains why the upper surface of its vault is very high compared to the surrounding ground. The fact that it is supported by the northern curtain wall, with a thickness of approximately 1.50m, is undoubtedly explained in particular by the resistance that this masonry could offer to the pressure exerted by the large mass of water. The cistern, supplied with rainwater coming from the roof of the building above it, had to be absolutely waterproof. To do this, the interior walls were covered with a coating 4 to 5cm thick. On the intrados of the vault, traces of the formwork which were used in its construction are still visible. Two orifices can be observed there. They were used to lower buckets using ropes to collect the water necessary for daily life. The limestone blocks delimiting these orifices still bear traces of friction caused by the ropes.
Despite the obvious signs of use of the building as a cistern, popular belief interpreted it at the beginning of the 20th century as being an oublice. It was even believed in the 19th century that it was a prison and that the prisoners were lowered into it through the openings mentioned above.
Now gunned and consolidated, the vault has been covered with dust to mark the ground level of the floor.
Poilvache 