The Great Cipher, created by Antoine and Bonaventure Rossignol was, at its time, one of the greatest ciphers ever made. Cryptanalysists for 200 years worked to break the secrets hidden in the Rossignol's cipher, but it was only until Étienne Bazeries, a French expert in cryptography, spent three years tirelessly working on this cipher that it was finally broken. At that time, cryptanalysists had discovered the secret of frequency analysis to crack monoalphabetic substitution ciphers and had even discovered how to decipher homophonic substitution ciphers by looking at the unique character each letter in the English alphabet has.
The Rossignols' cipher built on these techniques but made cryptanalysis exponentially harder by making a few adjustments to the old techniques. The Rossignols, instead of assigning multiple numbers to the most common letters as in homophonic substitution, assigned multiple three digit numbers to the most common syllables. This made frequency analysis, though not impossible, much harder and more complex.
Bazeries, who finally cracked the cipher, was only able to discover their methods after three years of many trials and errors. Bazeries first thought the cipher might be a regular homophonic cipher, then perhaps a similar cipher with pairs of letters represented by numbers. After many months of educated guesses, Bazeries finally was able to discover one word, "les enemis," by using advanced frequency analysis but with French syllables instead of words.
This cipher was much harder to crack than its predecessors because of the use of syllables instead of letters. The Rossignols complicated their system even more by adding some numbers as syllables and some as single letters. They also made their cipher harder to crack by adding "traps" such as a number that represented no syllable or letter at all, but rather the deletion of the previous number. By increasing the layering of their cipher, the Rossignols were able to create a very complex cipher that effectively kept secret the information for over 200 years. Though even the Great Cipher was not impervious to the scrutiny of cryptanalysists, and all ciphers will eventually be figured out, the strength of a cipher is not measured by if it is able to be broken but how long it keeps the information safe.