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!
Not an exact match, but sounds similar to The Beginning Place by Ursula Le Guin.
Just saw your response, almost a year later! 🙂 I will definitely check out that book. Thank you!