estre

"And tomorrow [...], shalt come over all the castle with me and see the estres and mark all its strength and weakness[....]"

--The Horse and His Boy, by C. S. Lewis, written around 1954; p, 239 of the 1994 HarperTrophy paperback edition

"Estre" isn't in MW3, but the 1913 Webster's defines it as "The inward part of a building; the interior."