One convenient method of introducing a reader to an imaginary world is through a Portal, which, for the protagonist, connects this world with the imaginary one. Hence Niels Klim falls down a hole in a cave to commence his Underground Travels (1741), Alice experiences a similar plunge into Wonderland in 1865, and Dorothy is swept by a whirlwind to Oz in 1900.
C. S. Lewis’s iconic wardrobe in the Narnia chronicles (1950–56) is perhaps the exemplar of this motif. For the reader, the Portal – whatever form it takes – cushions the transition from the familiar into the strange. A purely Secondary World such as Middle-earth has no need for such a mechanism; it is only really useful when the protagonist is from the Primary, or real, World. 

In some works, however, a character from this world accesses a Secondary World without any clear Portal, making the mystery of transition central to the narrative.

Image: Cover for ‘The Lion, The Witch and The Wardbrobe’, part of the first edition set of The Chronicles of Narnia by C. S. Lewis with illustrations by Pauline Baynes ©1950-56 CS Lewis Pte Ltd. Image courtesy of Ex Carta