When Douxie uses magic to make the broom sweep up for him, it is the same thing that happens in the poem The Sorcerer's Apprentice.
Inside the bookstore, there's a Guillermo Del Toro portrait. When the Green Knight attacks, the same portrait transforms in a Cthulhu like creature. HP Lovecraft is a big influence on Del Toro's artistic body.
Throughout the series there are numerous nods to the classic film Excalibur