Originally, the Murder Holes technology was meant to be in Age of Mythology, but was replaced with Boiling Oil for historical reasons.
Even if Boiling Oil has been researched, and the player deliberately attacks enemy units that are adjacent to an affected defensive structure, the player will still be prompted "(Building name)" is too close to fire!", despite the fact that the affected defensive structure already defends itself with boiling oil.
Enemies gathered at the base of a defensive wall could normally be attacked only if defenders leaned out from the wall and otherwise exposed themselves to arrow fire from below. One improvement for wall defenses was to provide boiling oil or other substances that could be poured on attackers below. Oil was particularly devastating if it could be set on fire first. This had the double effect of driving attackers away and burning their siege equipment.