The intact Wall of Illusion hiding Morphius' Citidel.
A Wall of Illusion is an impenetrable magical wall that can be created on entrances to caves, valleys or rooms to fool anyone who would happen to find one. The only thing that can destroy a wall of illusion is a Flying Disc of Frobozz. It is unclear who built the Wall of Illusion that concealed the Citadel of Zork in 1635 GUE but it is believed it was none other than Canuk.
Shattered Wall of Illusion.