Currently the message is a plain "Not while under attack!". This creates the impression of an oversight in explaining the game as the player probably should have known beforehand that it is not allowed to use the command while under attack. A better message would be "<some-monster> prevents you from ascending/descending the stairs with its vicious attacks!". In that way an out-of-game-education-problem turns into an in-game-experience-to-be-learned.