Donjon de Day, Medieval keep in Neuville-Day, France.

The Donjon de Day is a tall cylindrical stone keep with a steep conical slate roof, rising behind trees on a hillside above Neuville-Day. Its lower walls are pale, weathered masonry. A second turret with a spiral staircase flanks it, and a building with a Mansard roof abuts that turret. Inside are three round rooms, one per floor, with gun ports and arrow slits in the walls.

Tradition holds that Sigebaud, lord of Day and a companion of King Louis IX on crusade, built the fortified house from 1243. A storm damaged the castle on 25 December 1390. The Bohan family inherited it around 1430 and kept it for two centuries; an 18th-century fire destroyed much of the buildings. It was inscribed as a monument historique in 1987.

The first-floor guard room is covered by a seven-ribbed vault on crossed ribs and holds a large fireplace whose jambs are shaped like columns. Its mantel carries the arms of Gobert de Bohan and Isabeau de Ligneville, married in 1498; the Bohan held the castle for two centuries from about 1430.

The keep is a private property and cannot be visited; it is best seen from the road coming from Neuville, which curves around the hill below it. The tower stands at the top of a slope above the hamlet of Day, in the commune of Neuville-Day in the Ardennes.

A vaulted opening was found at the site, but it proves little about the supposed tunnel to the castle of the Marshal of France at Mont-de-Jeux. The story dates from the 17th century, when the seigneurie belonged to Marshal de Schulemberg.