My favourite part of any subcreation is its edges – apocrypha, marginalia, the sketches and hints at grander unrealised designs. Tolkien’s Middle-Earth is detail-rich (to say the least) but I’d linger fascinated on the Blue Wizards, or Rhun, or Far Harad: the stuff he left only as names. If anyone did fill them in I’d be mortified – this is one reason I won’t let my Doctor Who fandom take me as far as the novels, where mystery seems regularly to be given a thorough and pedantic kicking.

One of the funny things about these holes in a built world is that they work just as well – perhaps even better – if they arise from carelessness as if they’re planned. It’s a tightrope – revealing the mystery is bad, but knowing something was intended to work as a mystery can kill it for me just as surely. The Narnia series has a few of these – most of the close encounters with Aslan’s kingdom never impressed or moved me much as a kid because (I now suspect) there was too much narrative heavy-handedness around them.

But the series is also rich in moments where Lewis touches on a much vaster universe which seems all the stranger for being set against the slightly chintzy fairyland of Narnia. Tolkien apparently found fault with Lewis’ slapdash treatment of mythology in the stories but this is one of the series’ greatest assets. I read the books very young – there is a picture of me taken the summer Al was born, reading Prince Caspian; I would have been 4 and a half – and their sudden shifts in tone made pleasurable sense. Minotaurs on one page, Father Christmas on the next; jumping from the desolation of Charn to the violent slapstick of Andrew, Letty and Jadis in London.

The tedious modern reframing of Narnia-talk around the books’ Christian allegories tends to ignore this playfulness entirely. Almost alone among worldbuilders, Lewis is forever restlessly pushing at the boundaries of his own creation, and the more he does so the better the novels get. He’s impatient with Narnia itself. Pressed for a sequel to a hit children’s book, Lewis opens Prince Caspian by showing its world as a moss-covered ruin. Everyone the reader and human characters knew from that first story is dead: Lewis doesn’t make much of this fact but it’s there in the background. By his third book (publication order) he’s left Narnia behind and is sailing East. In the fourth he heads North (and Down), in the fifth South, in the sixth we hop worlds and end up at the beginning of time, and in the seventh we at last get a third whole book set in Narnia… only to see it destroyed. I don’t know how much this willingness to test his creation to destruction chimed with my tastes, and how much it shaped them.