I propose we play a game with some of the more eager residents. If they fail, it ends with a place getting attacked.
If they succeed, they flock to the location hinted at in the puzzle only to find that no attack is imminent to take place. At that point, it becomes clear that we've lead our players to hunt for the wrong solution to the puzzle and the real attack is to happen somewhere else.
They won't have time to scramble to the real location if we do it correctly.
[locked to idiot brigade]
If they succeed, they flock to the location hinted at in the puzzle only to find that no attack is imminent to take place. At that point, it becomes clear that we've lead our players to hunt for the wrong solution to the puzzle and the real attack is to happen somewhere else.
They won't have time to scramble to the real location if we do it correctly.