Frequency analysis
E is the most common letter in English — and that alone is enough to break any cipher that fails to hide the statistics.
The method of breaking substitution ciphers by counting how often each character occurs in the ciphertext and matching against the language's known distribution. It was described by al-Kindi in Baghdad in the ninth century, in what is generally counted as the earliest surviving text on cryptanalysis of any kind.
The analysis extends to bigrams and trigrams — "th" and "the" in English — and to word lengths when spaces are not concealed. The defence is to flatten the ciphertext's distribution: polyalphabetic ciphers, block ciphers with strong diffusion, or compression before encryption. The requirement on a modern cipher is stricter still — its output must not be distinguishable from random data at all.