Your story takes place inside of the mysterious upside-down Pyramid of Tutankhamen, a mysterious white marble monument created for the boy king's twentieth birthday. The main character is a female commoner of Ancient Egyptian society, who is desperately trying to save her tomb raider father from the impossible to solve labyrinth. Wearing a black cloak to conceal her identity, the character uses a hardcover copy of The Book of the Dead to solve riddles to find her way safely through the pyramid. She uses a makeup kit's face powder to avoid hitting detection lasers that would send her into a pit of man eating ligers. Also on her arsenal is a suit of armor, hidden beneath the cloak to fight the swarms of monsters and demons laying around the temple. Clutching her silver pistol, she rushes to the rescue of her single father, the property owner of their small farm following the divorce with his wife. It is one girl against the world.
Now that sounds like a promising story idea at first glance. It contains a female main lead, who uses guns and breaks the laws of Ancient Egypt in order to save her reckless father. This would possibly be an ideal fantasy and action story that readers would most likely with a little word of mouth enjoy. The idea of this story sounds promising except for one thing; the whole entire premise has the time period being represented wrong. King Tut never had a temple constructed for him due to his abnormally small reign, dying mysteriously at the ripe age of nineteen. There is no way he could have a twentieth birthday, let alone have a pyramid built when public knowledge already knew at the time it was not constructed at all. The main character would have no way of entering the pyramid because she is not royalty or a priest. Even if she did get in, she would not be wearing a black cloak and silver armor. Instead, the clothing would be lighter in color to avoid heat stroke and the armor would not be there as it was not invented yet. Her make-up kit would be nonexistent, as high quality make-up at the time was reserved for royalty, and the pistol would not even be close to being invented yet. Ligers, a hybrid lion and tiger breed, at this point would still be nonexistent. Texts such as The Book of the Dead would be on papyrus scrolls with only the few high class literate people in Ancient Egypt being able to have any use for it. Finally, for the biggest nail in the coffin, is the father. Tomb raiders for the most part got away with their crimes, there were no farms in Ancient Egypt, and if divorced back then, the property rights would actually go to his wife. The story falls apart with these series of historical inaccuracies.
You may be wondering why historical accuracy matters in fantasy at all. Is fantasy not the genre of free flowing imagination? While fantasy is meant for expressive creativity, it is important to know the setting of where the story is going to take place. If your story exists in a Game of Thrones like Middle Ages, having something like a car would stick out like a sore thumb. These kinds of instances can break the immersion your audience is getting from the tale due to the very evident lack of research to make the story seem more real. Something as small as giving your Ancient Egyptian lead a gun could deconstruct the story you worked hard to create. Even in a completely fictional setting, you need to know the time period it correlates to and how the people of this place live. It is the basics of fantasy world building.
Research is the key to creating an atmosphere towards your setting based on elements of the past. Without it, the audience is either going to be frustrated or will be asking a lot of questions from a series of historical inaccuracies. Even the smallest detail can matter.