We learn that the prince and his one thousand friends are in seclusion for five or sixth months before the prince throws his masquerade ball:

It was towards the close of the fifth or sixth month of his seclusion, and while the pestilence raged most furiously abroad, that the Prince Prospero entertained his thousand friends at a masked ball of the most unusual magnificence.

It is significant that Prince Prospero gives his grand ball while the plague is at its worst point. It is raging "most furiously" outside the castle walls, which means it is killing more people than ever. This implies that the especially magnificent ball is mean to take the guests' mind off the raging plague.

The prince and the guests have walled themselves up in his castle to try to avoid dying. The castle, we are told, has high walls and iron gates. Once the guests are inside, they weld all the doors shut, so that no desperate people can get in. With this array of walls, doors, and locks, the people inside hope to be safe from the ravages everyone else is suffering.

Of course, this is a selfish and futile way to live. Walls and bolts can't keep out trouble, and even the grandest ball ever can't keep the prince's privileged friends from being anxious about what is going on outside. They may have every luxury, but that can't stop them from worrying nor does it keep out the Red Death.