This doesn't work if the attacker knows the repeating pattern is going to be used, and if they know it's going to be either "ADVANCE" or "RETREAT" then it seems

This doesn't work if the attacker knows the repeating pattern is going to be used, and if they know it's going to be either "ADVANCE" or "RETREAT" then it seems likely they will also know about the repeating pattern. They can still invert the meaning by XORing the cyphertext with XOR("RRRRREEEEETTTTTRRRRREEEEEAAAAATTTTT","AAAAADDDDDVVVVVAAAAANNNNNCCCCCEEEEE").