Kingdom of Doorways
The Kingdom of Doorways is a walled realm entered through seven oak doors.
First named in The Cowardly Lion of Oz, ch. 5
"This is the Kingdom of Doorways," announced a large sign, posted half way up the walls.
Where this is
placed In Oz worked out
read out of the chapters cited on this page, not stated in any one sentence of them
Cowardly Lion of Oz ch9 places the travelers in the Munchkin Country while the Lion explains Oz's geography, with no border crossing between Mudge (explicitly 'in the Munchkin country of Oz') and this walled kingdom.
What the books say it is
appearance seven heavy oak doors set in the wall 1 chapter
And in the wall were seven heavy oak doors.
The Cowardly Lion of Oz, ch. 5