The key to the cipher (formed by arranging the original letters in alphabetical order of the substitutes) is "a verray parfit gentil knight", the description of one of the pilgrims in the prologue to The Canterbury Tales by Geoffrey Chaucer. This suggests a PILGRIMAGE made by knight's moves. All the unchecked letters in the down entries can be enciphered unambiguously with the exception of the N in 1 Down and the R in 17 Down; these signal the start and end squares of the journey corresponding to the approximate locations of London and Canterbury in a grid shaped like the southeast corner of England.