World War II gun turret in France

It was relatively isolated and came under heavy German attack, which inflicted a lot of damage and casualties. Requests to evacuate were denied by the French command, and as the Germans kept on shelling the place, a fire started, filling the structure with smoke. Every remaining defender who hadn’t been killed in the fighting died as a result of carbon monoxide poisoning. (apparently the levels got so high, they must have overwhelmed the gas masks most of the dead were wearing)

The Germans eventually went in with respirators, and found no survivors.