Tour aux Puces, Medieval fortified tower in Thionville, France

Tour aux Puces is a fortified tower in Thionville, the surviving keep of the castle of the counts of Luxembourg. Its exterior has fourteen corners, and it is built of irregularly large ashlar blocks, many reused from earlier buildings. A circular foundation about 2.5 to 3 metres high carries the polygonal elevation; stone corbels in the courtyard mark a former floor that has disappeared.

The tower stands on the site of a Carolingian palatium recorded in the mid-8th century and destroyed in the 10th; its north-east foundations date from about 1100. In 1292 it became the seat of the provostship, held until the mid-16th century, and around 1400 it was enlarged with new openings, a dividing wall and fireplaces. The Spanish rampart works of 1542–1558 folded it into the military domain, and vaulting is attested by a 1583 inscription.

Inside the tower, eight thematic rooms display regional objects from prehistoric times through the Renaissance period. These collections show how people lived in this area throughout different eras.

The tower stands on the Cour du Château, close to the Moselle in central Thionville. Its museum, open since 1966, occupies eight thematic rooms reached by a spiral stair to the upper floors. The base is a circular foundation 2.5 to 3 metres high, and the entrance is the rounded arched doorway at street level.

The French name Tour aux Puces mistranslates the Lorraine Franconian Pëtztuurm, literally "well tower". Archives record the building in 1295 as the tour de Mirabel or Meilbourg, and 19th-century locals called it tour de Thion. A local legend claims a fourteen-year-old princess was devoured by fleas in its dungeon.