This blog post was inspired by reading a book that had obvious debts to Roger Zelazny's Chronicles of Amber, and realizing that Amber is one development of the subgenre of portal fantasy.
Before I come back to that thought, though, I want to take a trip back to the origns of portal fantasy, and trace the highlights of its development through to the current small revival.
My theory of portal fantasy's origins - and I think it's a pretty well-founded theory - is that it comes to us via the influence of George MacDonald on C.S. Lewis, and ultimately has its roots in Celtic myths of the Otherworld.
As anyone who has dipped into Celtic myth will be aware, the Celts who made the myths that have come down to us regarded the stone "barrows" or burial mounds of earlier civilizations as the abodes of the faerie folk, and the entrances to them as entrances to the Otherworld, a world of gods or godlike beings, magic, and peril. Time ran differently there - a feature often seen in portal fantasy; an unwary kidnapped bard or other traveller might spend what seemed a night in the faerie mound and come out to find everyone he knew aged or dead. Rip Van Winkle is a version of this story, though the protagonist is Dutch rather than Celtic.
George MacDonald was thoroughly familiar with these legends (and many others, including the Jewish legend of Lilith, Adam's first wife), and used the idea of a magically-accessed Otherworld in two of his major works: Phantastes and Lilith. I have always considered Phantastes a dry run for the later Lilith; Phantastes isn't a bad portal fantasy, in my opinion, but Lilith is a great one, with a powerful theme of redemption. It would make a wonderful graphic novel, though the nudity would be a problem, given that its core natural audience consists mostly of Christians.
Lewis was quite explicit about crediting Macdonald as an influence, and I'm sure he read Lilith. The (legendary) Lilith gets a mention in The Magician's Nephew, in fact, as an ancestor of Jadis, later the White Queen (one can see Hans Christian Anderson's Snow Queen quite clearly in her literary ancestry as well; Lewis was nothing if not eclectic in his influences, something Tolkien apparently disliked about Narnia). Unlike Macdonald's Lilith, Jadis doesn't get a redemption arc; she is implacably an enemy, and perishes as such.
It was Lewis's Narnia that really popularized the portal fantasy, just as Tolkien's Lord of the Rings popularized the secondary-world fantasy (though in Tolkien's legendarium, Middle-Earth is not in fact another world, but an earlier version of ours, like the Hyborean Age of Robert E. Howard). Neither of them invented those subgenres, but they produced works that had such a powerful impact that many later writers either imitated them or took them as a point of departure.
Mostly, they imitated; the list of more-or-less straight knockoffs of Narnia and (even more so) Middle-Earth is too long to attempt to explore here. And mostly, with occasional exceptions, these imitiations didn't engage with the source material of their source material (the Celtic Otherworld in the case of portal fantasy - though that has been done, notably by Steven Lawhead - and Northern European myths in Tolkien's case). They turned what readers most enjoyed about those popular books into a set of largely unexamined tropes, much as we've seen recently with the rash of magic schools based, usually far too closely, on Hogwarts.
Those tropes include, for portal fantasy, the Chosen One(s) of prophecy, who comes from our familiar world (where he or she or they may be ordinary and little regarded, generally because of youthfulness), to become the only hope of saving the other world. This has inevitable colonial baggage, from a current perspective; it easily falls into the White Saviour trope, and declares pretty strongly that the other people over there can, at best, assist someone from our milieu in solving their problems. The strong presence of this theme may have been part of the cause for portal fantasy's waning popularity in the last couple of decades, though I think it was more likely just the unaccountable shifts of fashion that subgenres naturally go through. And, indeed, portal fantasy has never completely gone away, though it's more popular now than it's been for a while.
Later developments did eventually question and revise and reimagine some of the core portal fantasy tropes. Starting in the late 70s (and finally finishing only recently), Stephen Donaldson's long series about Thomas Covenant the Unbeliever takes an adult into a portal fantasy, and has him make things immeasurably worse, partly because he doesn't believe it's real, but largely because he's a mess of a human being. Philip Pullman's His Dark Materials series is explicitly a reaction to what the author hated about Narnia, especially the Christian elements. Foz Meadows' recent Manifold Worlds series takes place in a portal-fantasy setting largely imagined when she was in her teens, and full of elements that represent wish fulfilment for a queer teenager; this inevitably makes it quite different in emphasis from Lewis and his imitators. Jo Walton's short story "Relentlessly Mundane" (collected in Starlings) and Seanan McGuire's Wayward Children series explore what happens to the children when they come back to our world, having experienced a very different life where they matter a great deal and are not ordinary, and attempt to explain themselves to the parents and other family members they've left behind.
And, as I realized this morning, Zelazny's Amber is in the portal fantasy tradition too. It didn't immediately jump out at me as a portal fantasy, because, in a bold Copernican move, our world is decentred; it's only one of many "shadows" cast by the true world of Amber, and only as real-seeming as it is because Corwin, a Prince of Amber, has spent so long here. Zelazny takes up the multiversal idea that's implied in portal fantasy, and occasionally made explicit - for example, in the Wood Between the Worlds in Lewis's The Magician's Nephew - and puts it at the centre. (My only work of portal fantasy, the short story "Gatekeeper, What Toll?", was directly inspired by Zelazny, and goes a step further, putting the portal itself, and not the multiverse, in the centre.)
There's an obvious appeal for children and adults alike in the idea of a different and more wonderful world, where people like you or who you can identify with are more important and significant than you are, and have thrilling adventures. Really, a lot of genre fiction, portal fantasy or not, is working in the same emotional territory, but portal fantasy explicitly offers the chance that we might escape, even if only for a time, from our mundane lives and problems to somewhere more colourful and exciting (though also more dangerous). Some of the questions it helps us to answer are: Who else might I be or become? Can I be a leader, a hero, a worker of wonders? What kind of companions do I need to achieve that? And it takes us somewhere strange and wondrous, which is enjoyable in itself. And then it brings us back home, and sometimes it even asks: what now? What changes in our world because of what we've experienced elsewhere? How do I be, in my mundane life, the person I learned to be in the Otherworld?
One way you could look at it is that fantastical literature is itself a portal to other worlds of adventure, in which we can explore other identities and roles for ourselves. And as more perspectives get admitted into the literature of the fantastic, so the places we can explore and the roles we take on only become more varied and interesting.