What words or phrases does the author use to support the idea that the guests are well cared for and protected?

In order to suggest that Prince Prospero's guests are well cared for, the narrator describes the "castellated abbey" to which they've been invited.

[It] was an extensive and magnificent structure. . . . A strong and lofty wall girdled it in.  This wall had gates of iron.  The courtiers, having entered, brought furnaces and massy hammers and welded the bolts.

There is, therefore, no means of entering or exiting the castle, and the guests feel...

In order to suggest that Prince Prospero's guests are well cared for, the narrator describes the "castellated abbey" to which they've been invited.

[It] was an extensive and magnificent structure. . . . A strong and lofty wall girdled it in.  This wall had gates of iron.  The courtiers, having entered, brought furnaces and massy hammers and welded the bolts.

There is, therefore, no means of entering or exiting the castle, and the guests feel secure that nothing and no one will be able to find a way in.  In this way, they hope to shut out the outside world and its contagion. They also hope to maintain their own health inside the castle walls.

The narrator also describes the interior of the castle as a way of showing how secure and safe the guests feel.  "The prince had provided all the appliances of pleasure."  There are dancers, musicians, food and drink, and beauty. All these and security were within.  Then, after five or six months of this seclusion, the prince throws a huge masquerade party for his guests.  He takes great pains to create an environment that is both luxurious and fantastic, although he and his guests cannot, ultimately, escape danger forever.