Why is this the answer?

The answer is egg because this is a truly creative riddle that paints a beautiful picture of something you might eat for breakfast! This classic puzzle uses special clues to describe all the different parts of an egg. Let's look at the first line: "In marble halls as white as milk." The riddle is talking about the shell of the egg. A hen's egg has a hard, white, protective shell. It is like a "marble hall" because it is a sturdy, white, round building that holds something important inside. Next, the riddle mentions the hall is "lined with a skin as soft as silk." Right underneath that hard, outer shell, you can find a very thin, soft layer that protects the egg even more. If you have ever peeled a hard boiled egg, you have probably noticed this thin membrane. The riddle compares this fragile inner layer to smooth, soft silk, which is a wonderful clue. The third clue mentions, "Within a fountain crystal-clear." This clue is pointing to the egg white. The white part of the egg is mostly water, and when it is raw, it is clear and liquidy, just like clear, sparkling water in a "fountain." This clear liquid surrounds the yolk. Inside that clear fountain, "A golden apple doth appear." This is one of the best clues. What part of an egg is golden and round like an apple? That is the yolk! The yolk is the special yellow center of the egg, full of good things, and the riddle calls it a precious "golden apple." Finally, we have the last lines: "No doors there are to this stronghold, yet thieves break in and steal the gold." An eggshell is a "stronghold" because it is a sealed container protecting the inside, and it definitely does not have any doors. The "thieves" are people, and we "break in" by cracking the shell open. We do this to get to the yummy yolk, which the riddle beautifully calls "the gold." This classic riddle uses those secret code words to perfectly describe a simple egg from start to finish.