One information that is truly important when conducting Cryptographic Attack, it is: the nature of the plaintext. Is it a sentence of a particular language? or is it of some software code? If we know the fundamental nature of the plaintext, then our code breaking job could be a bit easier.

If it is of a commonly used language, then we can use the very unique characteristic of that language to help deducing the plaintext. One popular method is frequency analysis. In the languages of Latin origin, we know that certain alphabets are of higher frequency of appearance than others.
In English, we should know that the vowels – ‘a’, ‘e’, ,’i’, ‘o’, ‘u’ appear more often than other alphabets.

And if we run analysis of a particular language long enough, we can deduce a table of relative frequency of each alphabet appearing in that language.

So if a particular language message is encrypted using substitution method like Caesar Cipher, we can easily break the code using frequency analysis if we know the original language used in the plaintext.