I am searching for a book probably written very early 60s. Its about a girl that goes through a gate or door in a hedge and finds fairies. She goes to play with them every day. I think her name was Nan and I think the book had castle or manor in the title but I could be wrong about this. Ive searched and searched with no luck. Its not The Secret Garden.
Lost Classics of Teen Lit: Name That Book! 
Your description doesn’t quite fit this book, but I throw this out just in case this is the book you remember. I highly recommend this:
Shadow Castle
by Marian Cockrell
Published 1946. “In the middle of a deep forest is an enchanted valley and a castle where only shadows live, shadows of kings and queens who have waited for hundreds of years for the spell cast upon them to be broken. One day, a girl named Lucy follows a little dog through a tunnel into the valley…”
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