This is a beautifully illustrated, middle-reader American (I think) fantasy book about a brother/sister or possibly male/female friends who somehow stumbled from their somewhat urban home/apartment building into a medieval setting when coming home from school one day in the woods near their home.
I was born in 1970 and was a very early and voracious reader, so this book cannot have been published much later than 1976; my guess (contextually) is that it’s most-likely a little earlier than that.
Many thanks!!