The caesar square is a transposition cipher.
The plaintext is always in groups of a perfect square such that for any number n^2, it will give you a square box that is n characters tall and n characters wide. You then write from top to bottom, initially and left to right after each column is full.
For example, the phrase:
thekinghasbeenkilledbytom
is 25 characters long.
therefore, our ciphertext looks like:
tnbibhgelyeheltkaneoiskdm
Encoding it is simple. Since we know the string is 25 characters long, we know our box must be 5x5 characters (as 5X5 = 25). We then write it in the box like so:
tnbib
hgely
ehelt
kaneo
iskdm
and we can get back our original message, if we read top to bottom.