The female main character, Nan, lives in an apartment in a big city in what I recall as the late 1970s or 1980s, although it could certainly have been as late as 1990. In my memory, she finds a doorway to a mountain world at the end of an alley. I remember the vast majority of the book is spent with her alone, exploring the mountain.
I seem to recall that near the last 1/3 of the book, she encounters a village. I believe she also travels back and forth and at some point, decides to leave her boyfriend in the city life. However, I am fairly sure that the bulk of the book takes place on the mountain. I recall grassland vividly, so description of the environment must have been a big component of the book.
Near the end of the book, there is, of course, conflict. I can’t remember if something gets in to the mountain world and upsets the balance or if there is some sort of struggle on the mountain. I vaguely recall a fire.
I checked out this book from the library in 1993 or 1994, and because of the female protagonist, I believe it was written after 1970.
Thank you so much for your help!