The Polybius square is not the most secure cipher; there are few advantages and strengths in when using the Polybius square. One of these few is if the numbers in the message when encrypted are left with no spacing in between, the interceptor will have trouble finding which numbers are separated. However, the harder for the interceptor the cipher is, it will be just as hard for the recipient.
Weaknesses of the Polybius Square
Unlike most ciphers, the Polybius square does not have a key. Also, not many ciphers do not convert their letters to numbers, or at least permanently, while also none of the numbers go above five (or six). So if a person gets a hold of a message that is all numbers, he can assume that the cipher used was the Polybius square.