using the wall in this way, Gordimer makes an important point about home security and inflated fears of the outside world, that is, that we must not let our paranoia and fear dominate our way of living. Not only will this destroy communities, but it will, quite literally, destroy our lives.

The wall can be seen as a symbol in a couple of ways.  The first is that a wall, itself, is a type of barrier.  When I think of a "wall," I think of a partition, a form of dividing oneself or elements from one another.  Certainly, the wall can be seen as representing this in the story, as the family, specifically the mother and father, become obsessed with keeping their family safe.  This vision of "safety" is one in which the outside world is repelled, kept at a distance.  The family recognizes this as the outside world is seen as one rooted in lawlessness and chaos, elements that the family wishes to keep away from their own consciousness.  The wall is a part of that.  The fact that the wall itself was designed to keep the outside world away from the family, or keep the family cut off from the outside world is representative of this idea of partition and separation.  This is enhanced by the shards on the top of the wall, designed to prevent anyone or anything from seeking to climb it.  Finally, the wall's addition of the barbed wire trapping on the top of it is the final element that displays to what extent the family will go to ensure that the outside world is "walled" off from them or that the wall itself keeps the family "walled" off from all that is on the outside.  It is here where the wall operates as a symbol, a representation of how alienated the family ends up becoming both from the outside world and their own sense of happiness as demonstrated with the ending of the story.

In the story, the white homeowners become fearful of the "people of another color"—Black people—who engage in riots elsewhere in the city, away from the suburbs where people of color are only allowed in as "reliable housemaids and gardeners." They install a set of electronically controlled gates to the house, on which is posted a "Neighborhood Watch" sign, but after another home in the development is burgled by persons of color, they decide to install an alarm system like many other white homeowners in the area possess.

Soon, the thieves simply ignore the shrill alarms and use the sound to drown out the sounds they make breaking glass or sawing through window bars. So the family decides to build the wall around the garden higher so that no "tsotsis," or hooligans from the street, can get in and threaten the family's safety. Further, the eventually top the wall with barbed wire.

This wall is the family's literal attempt to protect themselves from the outside world, from the people of color whom they see as dangerous thugs rather than human beings who are forced to resort to any means they can to survive. Figuratively, it represents their privilege: they have power and status, they can choose to use their money to buy expensive liquors or ridiculous alarm systems, and they can look away when they see people suffering, because they are not suffering themselves. The wall symbolizes their intentional, willful ignorance of the conditions in which persons of color in their community live and die.

In "Once Upon a Time," Nadine Gordimer uses a razor-wire topped wall around a single-family home as a metaphor for South Africa's apartheid. Like the family in the story, which is trying to stay "safe" by turning its room into a fortress, the white minority in that country was, at the time, trying to keep itself "safe" from its black majority by strictly separating blacks and whites. Blacks had to live in different areas, attend different schools, and were cut off from most of the amenities of modern life, while the whites lived in great comfort.

Gordimer very much opposed apartheid, and the story becomes a cautionary metaphor for how the entire white society is going to end up ripping itself apart, destroyed by social structures meant to keep it safe, just as the little boy is ripped apart by the razor wire that is supposed to keep him safe.

The motif of the fairy tale is another metaphor for South Africa's apartheid. Gordimer is likening the way the whites live to living in a fairytale. All the safety and security they think they are buying for themselves is nothing but an illusion. Just as the family is less safe because of the razor wire, so white society is less safe, not more, because of the cruel social structures it has established to keep blacks out.

This metaphor can be extended universally, as the story is carefully not set in any particular place and time. Is building a wall to keep a feared 'other' out ever going to make people safer or is that whole idea a fairytale illusion?

The most dominant metaphor in the story is the sign on the house.  I feel that this is used as a constant reminder to so many.  The family use this as a warning to potential intruders.  Yet, this sign could also be a warning to the family that allowing their fears and paranoia to drive their consciousness is disastrous.  The idea of "YOU HAVE BEEN WARNED" is a metaphor, a comparison, of how the family refuses to accept their own fears and embrace them as part of being in the world.  Rather, it is an object that represents both the desire to project strength as well as a reflection of internal fear.  I think the sign is a metaphor because it compares the family to the outside world and also simultaneously internalizes the outside world in the family.  Another metaphor would be the wall, representing the "walling off" of the family from the outside world and also preventing them from fully understanding their own consciousness in this world.  It makes sense that this is the object that the little boy tries to climb, as he seeks to overcome this metaphor of isolation, only to be trapped on the top of it and die because of it.  A potential metaphor could be the frame story that serves as the exposition, in that it shows how different an appropriate response to fear can be.

Given the way in which the story is framed by the author, and given the very short length of this story, we can safely infer that many of the objects and perhaps even the people within the fairy tale can be meaningfully interpreted as symbols. Let's look at each:

The wall: Probably the most important symbol in the story, the wall represents separation and intolerance, the "closing off" of one's mind. When you place this story in its historical context, the symbol becomes even more important: the author seems to be hinting that the social separation of apartheid cannot be tolerated, that it only leads society to destruction.

The boy: When you consider how he gets ideas from his reading, tries them out, tries to get out and over the wall, and ultimately is destroyed, you might argue that the boy represents some or all of these ideas: youth, open-mindedness, experimentation, exploration, reason and rational thinking, and the striving for openness and equality.

The maid: Take a look at every time the housemaid is mentioned, and notice how she's never just "the housemaid." She's always the "trusted housemaid" or "the reliable housemaid." The implication is that the family believes that most people the same color as the maid are not trustworthy. Combined with the fact that the family's "YOU HAVE BEEN WARNED" sign is carefully constructed so as to not reveal the race of the pictured intruder, we know that they're eager to distance themselves from the notion of racism. The maid, then, possibly represents superficial, ineffective, or insincere efforts to eradicate racism.

The wife: She's the embodiment of both positive ideas (compassion, love, and the desire to protect one's family) and negative ones (excess, complacency, ignorance, obliviousness).

The mother-in-law: As the closest thing that this fairy tale has to a villain in human form, and the oldest character, this "witch" might represent tradition, or the blind adherence to tradition. (The family simply does what the mother-in-law says without question and accepts the gifts she gives, again without thought to their possible consequences.)

Notice how I kept saying that something might represent something else, maybe, possibly, could, and so on. Unless you can find an explanation straight from the author about what's supposed to represent what, you really can't say for certain what one thing symbolizes; you can only make a good argument based on what you see in the text and what you know about the larger context surrounding the story (like the author's biography, where she lives and what was going on socially and politically at the time, and so on). 

Complicating matters further is that, when we’re talking about literary symbolism, any particular thing usually represents more than one other thing. (A story in which every object only represents one definite thing is an allegory.) Other readers may be able to think of many other possible answers to add to this list!