The cryptography process of the Ceasar Cipher is frequently incorporated as part of more complex schemes and continues to have modern applications, as in the system ROT13. Like all monoalphabetic substitution ciphers, the Ceasar Cipher can be easily decrypted and, in practice, doesn't offer much safety in communication.
According to Suetonius, Julius Ceasar used the three positions trade cipher to protect messages of military importance. Although the use of this scheme by Ceasar is the first one to be registered, it is known that other substitution ciphers were used previously.
It is impossible to know if Ceasar's ciphers were effective during his time, but it was probably reasonably safe, given that most of Ceasar's enemies were illiterate and others presumed that his messages were writen in a foreign unknown language.