The inhabitants of the city all die of the Red Death plague.
When the Red Death falls upon Prince Prospero’s city, many people die. The Red Death is dangerous and deadly. It wipes through the population so quickly that Prospero doesn’t bother to help his people. He just flees, locking himself and his thousand closest friends in a fortress.
But the Prince Prospero was happy and dauntless and sagacious. When his dominions were half depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys.
As you can see, half of the people in the kingdom had died by this time, and the rest were well on their way. The Red Death killed by liquidating its host. It was highly contagious and there was no way to escape it.
Prospero and his people thought they were safe inside their castle. They felt that if they barricaded themselves away and did not let any infected people in, they would not be infected. This was a plan that did not succeed. Even though they survived six months this way, the Red Death still found them.
And now was acknowledged the presence of the Red Death. He had come like a thief in the night. And one by one dropped the revellers in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall.
When Prospero first saw Death, he does not believe it. He is convinced that the imposter is just playing with them, dressed in a morbid costume. He tries to fight back, but it’s pointless. He dies, and so do all of his guests. They die instantly, unlike the other victims.