Trying to finish a crossword with no mistakes is hard(both)

One hobby of mine is Crossword Puzzles.
I buy Mr Wisdoms Whoppers Crossword book.
27 Crosswords, Average of 400 clues per puzzle.
I have been doing these puzzles for 20 years and have never completed one in toto.
The last one done, 3 spelling mistakes, 6 unknowns.
Who would know what a Medieval Castle Privy was called?
9 letters, _ a r_ e_ o b e