Château de l'Engelbourg, Medieval castle ruins in Thann, France

Château de l'Engelbourg is a ruined summit castle on a hill above Thann. Its oval enceinte, about 150 m long and 70 m wide, sits at 445 m altitude on terraced slopes. The most visible remnant is a pale grey masonry ring with a round hole, the broken keep called the Witch's Eye, standing above lower wall sections and foundations.

The castle was built between 1220 and 1230 by Ludwig III of Pfirt to control the entrance to the Thur valley and its toll route. It passed to the Habsburgs in 1324 through the marriage of Johanna of Pfirt to Duke Albrecht II. Louis XIV ordered its demolition in February 1673, carried out by miners from Giromagny under intendant Poncet de La Rivière.

Locals call the broken keep l'Œil de la Sorcière, the Witch's Eye, because the fallen tower left a round hole through the masonry. The name Engelburg was first recorded in 1465 and may come from a castle chapel dedicated to the Archangel Michael. The chapel mentioned in 1304 was dedicated to Saint Catherine.

The site is open to the public and offers wide views over the valley, so sturdy footwear is advisable. The paths are uneven and sometimes steep, requiring a bit of effort to explore.

During demolition, a tower collapsed in an unusual way and left a circular opening now called l'Œil de la Sorcière. This odd hole remains one of the most visible features of the ruin today.