The following message was encrypted using a Caesar cipher with a key of 2: fghgpf vjg ecuvng. What is the plaintext message?

defend the castle

Explanation:

Caesar cipher is a substitution cipher in which each letter is substituted a certain number of times (in this case 2) down the alphabet. Given a key of 2, we reverse this by going up the alphabet twice for every character since we are deciphering the cipher. so 'f' will be 'd', 'g' will be 'e' and so on