This fantasy genre takes a historical period/event and rewrite it to include fantasy elements. For example, the story may talk about how the goblins played a hand in building the Eiffel Tower.
Different worlds are hidden within or parallel to our own as in Gulliver's Travels by Johnathan Swift.
Some worlds are to be found through a mirror as in Through the Looking Glass by Lewis Carroll.
The heroes encounter figures from history, literature and mythology as in Silverlock by John Myers.
One sub-genre of alternate history is steampunk. The action is set in Victorian or Edwardian era and includes steam-powered technology linked to magical beings. Steampunk features technology that is advanced for its time, but bulky and noisy. Rust, grime and grease come out of the gears, pipes and cylinders.
Example: City of Ember or movie Treasure Planet.