The Rail Fence Cipher (alternatively known as the Zigzag Cipher) is a transposition cipher. The unencrypted plain text is written downwards and diagonally to the right on lines or "fence rails" and then switching upward once reaching the bottom. The pattern is in a zigzag pattern (see photo example below). The message is then read off in each row as an encryption.
Plain text: DEFENDTHEEASTWALLXX
Encrypted text: DNETLEEDHESWLXFTAAX
nheoeghznwonurnrnoottrewtreihtreeoieet
noeorheoeheoewrhtheternseftetet
You can validate your puzzle solution with certitude.