"The Crossovers"

  Earth steel is stronger than any metal on the other planet.  This fact also contributed the lack of technological advancement on their planet, which also contributed to their greater focus on mental abilities.

  Long ago, a group sought a better technology, a better material.  They focused and a portal opened in England, on earth, when the use of steel had gotten its foothold.

  Why England and why at that time?  Part was sheer coincidence.  They were not able to travel through time, but at that time, England offered the greatest balance of availability of the technology and the greatest safety with which to infiltrate the world and take some of it.

  Earth steel is extremely valuable and highly sought after, but is relatively rare.

  Over time, other trips were made.  There were human encounters ranging from being seen briefly by humans to the more rare diplomatic meeting.  Some humans even went to the other world with them: some remaining for life and some returning later.

  These encounters birthed many myths, legends and fairy tails:  Some with a base in reality and some with human imagination getting carried away.

  Some groups focus a lot of time and attention on martial skills.  One dedicated group sought someone who could teach them something new and help them excel in what they already knew.  Eventually, they were able to open a portal to Miyamoto Musashi: Earth's greatest swordsman.

  This was after he had left his wilder ways; when he had sequestered himself to read and further his mental discipline, but more importantly, was of more of an accepting mentality, willing to go and teach.

  Since you can spend a year on their planet, and only a week has passed on Earth, Musashi was able to spend a lot of time with them, and he needed it, given the language barrier.  Some of each other's language was learned, but they came to discover that not all that much speaking was required.

  Another human visitor was C. S. Lewis.  He was not sought, but he did have an encounter and visit briefly.  During his visit, he had quite a scare when he and his hosts had an encounter with an exceedingly dangerous breed of forest creature called the Jaba Wokai.

  Jaba Wokai is a creature that many doubt exist at all.  Standing fully upright, the adult reaches roughly 10".  It can walk on two or four legs, though the front legs do more resemble arms in comparison.

  It is quick, agile, and stealthy and has terrifying teeth and claws.  Its sense of smell is superior.  There is no greater known predator.  Fortunately, it avoids people if possible.  It is a social creature among its own kind and has an intelligence level slightly above a dog.