Worldbuilding is tricky business. You need to convey the unique fictional world you’ve created without dumping a bunch of information inelegantly in your reader’s lap. You have to give them enough context to understand what’s going on and to make sure that the framework, boundaries, rules, and unique qualities of your universe are conveyed clearly.
Let’s say that people can fly in your world. This isn’t a unique premise but it is your premise, and that’s what matters. Now, let’s say that you choose to hold off on this fact and use it as a reveal at the climax of the book. The character has no idea that people can fly in this world and only learns it at one of the last moments. Thrilling, right? Well, maybe. If it’s done right. But if this is a world where people can fly, why save that unique tidbit until the very end? Why not blow your character’s mind right at the beginning and get more mileage out of the flying than you would if you hid it away?
Your job is to attract readers to the world you’ve created by giving them something that will get them interested in your unique idea. You certainly can tease and hint and withhold things about your world, but I would do this sparingly. Instead of counting on a big surprise to raise stakes and elevate tension, get the coolest stuff about your idea out in the open early.
Instead of hiding your world, SELL IT to your readers by dropping clues for them to follow or exposing the elements that made you fall in love with your story and pursue it. This is a great way of drawing in your audience. It’s saying, “Sure, you’ve read a lot of fantasy before but MY fantasy world has people flying, and glittering unicorns, and a giant who only falls asleep while guarding his cave of precious treasure once every hundred years.”
The more we’re in that world and understand how it works–all of which takes information and revealing these elements in a timely manner–the more we can focus on the other elements of your storytelling. Compared to a rich world full of interesting elements, the cheap fizzle of a last-minute surprise starts to feel like a bummer.