The primitive castle was destroyed, sorry... Indeed, completely burnt during the battle of Villersexel, which took place in the neighbouring plain: French and Prussians fought there, on January 9th and 10th 1871. The French won... But it was the only French victory in this war opposing France and Prussia!
So, the city of Belfort was, in Eastern France, the only place which remained French. Belfort, bravely defended by the famous colonel Denfert-Rochereau... So, the French decided to gave all they’ve got and sent general Charles Bourbaki, leading an army of 12 000 soldiers. To save Belfort from enemies! But on the way, they came up against Prussians in the plain of Villersexel! The battle lasted 2 days.
There were huge numbers of dead: 4000 for Prussia, not to mention the prisoners. What about the castle? It fell into the hands of Prussians. The French had to turn them out! They shot at them and they finally left: but they started a fire... So, the current castle dates back to 1885, re-raised after the battle. As for the former one, you’ll see it near the river Ognon, in the park.