so̞to̞ no̞ se̞ka̠i
The Outside World (外の世界 soto no sekai, also called the "Human World", "Real World" or "Our World") is, in reality, the non-fictional world of fact and reality we all live in. In the Len'en Project, the outside world is referenced by its residents, where it's location is outside Mugenri. All its residents are aware of the outside world, whereas on the other hand it's presumed that almost all the creatures of the outside world are unaware of the existence of Mugenri as the Mugenri Barrier keeps it all sealed away. It's thought that no youkai live in the outside world.
The outside world is very likely to be based on the Outside World of the Touhou Project . The Outside World of the Len'en Project was described to be essentially the same as the real world that we live in. Even though it's impossible to pass through the Mugenri Barrier, it's still very easy to enter by simply getting lost while wandering around, but humans are still unaware that they've entered another world. This leads them to great danger. When JynX was asked if both Touhou and Len'en's Outside World are the same, they could not give a definite answer due to canon issues when it comes to games they did not create, and thus said to "leave it to your imagination."
Even though Mugenri is a world of fantasy and the outside world is a world of reality, various outsiders are shown to have abilities most outsiders don't have. These outsiders in question were spirited away to Mugenri by Tsurubami Senri in Evanescent Existence. As such, the main protagonists of the Len'en Project are outsiders, who are Yabusame Houlen and Tsubakura Enraku. The rest in said game are Kurohebi, Aoji Shitodo, Hooaka Shitodo, Kuroji Shitodo and Clause.
Some characters have the capability to travel between Mugenri and the Outside World: Tsurubami (Mugenri's shinto priest) travelled to the Outside World for a holiday, Yabusame can cross dimensions, Tenkai Zuifeng can manipulate barriers and Suzumi Kuzu has their own secret way. Their ways of travelling between the two appear to be easy, but according to JynX, it's easy to travel to Mugenri, but harder to get back out. Tsurubami, for example, couldn't send the humans in Evanescent Existence back to the Outside World because it was difficult to send them and themself over the Mugenri Barrier, even by their standards of power. They cannot go in and out as they please.
- Evanescent Existence
In Evanescent Existence, Tsurubami Senri spirited away various outsiders to Mugenri so they can go on holiday and have someone else take care of their youkai extermination job. As such, these outsiders thought that whoever reaches the "goal" first may attain something and to escape from. However, no escaping was shown and it's presumed that all the outsiders settled in Mugenri.
- Reactivate Majestical Imperial
In Reactivate Majestical Imperial, winter is lasting long and a huge amount of spirits were coming from the Outside World. This was because Tenkai Zuifeng pulled a prank on their superiors and messed around with the Mugenri Barrier, which turned out to not be a good thing.
- Touhou Wiki's entry for Outside World