What is “murder holes” in a gatehouse used for?

The story behind the answer

In medieval gatehouses, “murder holes” were openings in the ceiling of the entry passage that let defenders attack enemies trapped below, often between raised and lowered portcullises. From these apertures they could hurl stones, pour boiling water, heated sand, or lime, and shoot downward into a tightly packed kill zone where attackers had little cover.

They’re related to machicolations, which project outside walls to target foes at the base of the curtain; murder holes, by contrast, are inside the gateway. Despite popular myths, boiling oil was rarely used—too costly and dangerous to handle—while hot water and sand were common, effective, and readily available.