I read this story in High School, sometime before 1993, in English in the USA.
Unfortunately, I don't remember much about it at all, other than that I loved it and almost panicked after I returned it to the library, because I had immediately forgotten the title and author and couldn't ever find the book again!
It is not the Doonfarers of Coramonde by Brian Daley, although that has been suggested as the solution to me more than once over my years of looking. I'm like 97% sure of that.
I do not recall if this is a short novel or a short story from an anthology, although my impression is that it was a short novel, of the yellow-spined DAW Sci-Fi type.
What I remember of the story is that a person (maybe more than one person?) from the modern world was transported to a misty realm where legendary, mythical cities from Earth existed - Avalon and Tara are the two whose names I can remember, but there were more. Maybe seven.
There was a scene where some modern-ish (but outdated - possibly time-displaced Viet Nam War-era) soldiers showed up briefly and got into a firefight with some mythical creatures - one was attacked and impaled by a Unicorn.
And that's it - that's all I remember. I know it's not much to go on, and I don't remember the beginning or the end, but I'd love to find it again. Hopefully those clues are enough to jog someone's memory!