Question 1

Where is Sanmon?

Accepted Answer

Sanmon is in Minobu, Yamanashi, Japan. You can see its exact position from the map coordinates and the Street View link.

Question 2

Which scene of mono does Sanmon appear in?

Accepted Answer

Sanmon appears in EP7 of mono.

Question 3

Can I visit Sanmon?

Accepted Answer

Yes. Sanmon is a real place you can visit. Check what it looks like on Street View before you go.

Question 4

Where was mono filmed?

Accepted Answer

Anime and manga are not filmed the way live action is. Their backgrounds are painted from photographs of real locations, and Sanmon is one of those real places (EP7).