The Ambush tile is an invisible tile that may be placed in maps in Wolfenstein 3D and its ports. Any enemy placed on an Ambush tile will be deaf to all attacks - whereas a normal enemy responds immediately to any gunfire or screams in the room, one can quite literally shoot guns immediately behind a deafened guard without attracting his attention.
An ambush tile-affected enemy is not blind, attacking the player on sight or upon being attacked himself. However, he is quite stupid, as even the sight of a fellow Guard falling dead of gunshot wounds will not alarm him, as long as he doesn't see the player.
The reason for an Ambush tile-affected enemy's deafness is explained as his training:
"Each enemy is given specific orders which dictate his actions once he knows of your presence. Some are ordered to immediately attack, while others are trained to act only upon visual contact."