Julius Caesar probably wrote it to keep his instructions to the minister from being peeked.

When writing, be careful that your handwriting must be completely square, so this is a bit troublesome.

Then, write what you want to write vertically from left to right, and copy what you want to read horizontally from top to bottom. This is ciphertext.

I have done (plaintext) IVOHENADE (ciphertext)

I'm Zhang ta

See you at the third branch

A de de zai Yu mian

Zhang San met him in the hotel (in plain text) and Zhang Jiu met him in his third shop (in ciphertext).