Sir John Trevanion was a prisoner at the Colchester Castle during the English Civil War, but managed to escape thanks to a letter which hided a message "PANEL AT EAST END OF CHAPEL SLIDES", which showed him an escape route.
Although this episode is famous as a spectacular feat of steganography, nobody seems to know a primary source of this episode, as I pointed out in 2012 in "Quest for 'Trevanion's Cipher'".
I made some additions to the page (references from 1878 and 1925 and some peripheral information).
I have now found the 1922 archaeological paper about the discovered narrow stairway I mentioned in the article. No new information.
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