A fictional world that characters from our reality (or another known world) cross into, usually through some doorway, passage, or magical transition.
A portal world is a secondary world that characters travel to from a different, usually familiar, setting. The "portal" can be literal (a wardrobe, a rabbit hole, a mirror) or metaphorical (a sudden transformation, waking up somewhere else). The key structural feature is the contrast: readers experience the new world through the eyes of someone who shares their frame of reference, which makes exposition feel natural because the protagonist is learning alongside the audience.
Portal worlds solve one of the hardest problems in worldbuilding: how to introduce a complex setting without drowning the reader in exposition. Your fish-out-of-water protagonist asks the questions readers want answered, and their wonder (or confusion) becomes the reader's entry point. It's a powerful structure for making alien settings feel accessible.
The wardrobe is the iconic portal. Lucy's discovery of Narnia through an ordinary piece of furniture defines the entire subgenre.
The rabbit hole is a proto-portal. Alice's bewilderment at Wonderland's nonsensical rules mirrors the reader's own disorientation.
A tornado carries Dorothy from Kansas to Oz, and the stark contrast between the two worlds (gray vs. technicolor) became a storytelling template.
An early portal story where a modern person's knowledge clashes with the world they enter, creating both comedy and commentary.
Get to the portal quickly. Readers picked up your book for the fantasy world, not for fifteen chapters of ordinary life. You can always fill in the protagonist's backstory once they've crossed over.
Being ripped from everything you know is traumatic, thrilling, or both. Let your character actually process the emotional weight of crossing worlds, not just marvel at the scenery.
Just because your character can ask questions doesn't mean they should sit through a lecture. Spread the answers across the story and let some questions stay unanswered for a while.
Write the moment of crossing. Your character steps through a portal and their senses are hit by a world that works differently from ours. Describe their first 60 seconds in the new world using only sensory details: what they see, smell, hear, feel, and taste. Don't explain anything; just let the strangeness land.