Why are old castle walls topped with zigzag battlements?

Answer: Defenders shoot from gaps, hide behind solid parts

Defenders shoot from gaps, hide behind solid parts ✓ — Correct! The solid blocks (merlons) give cover; the gaps (crenels) are firing positions. An archer steps into the gap to shoot, then ducks back behind the merlon to reload — a 1,000-year-old version of the pop-up cover used in modern combat. Universal across China, Europe, and the Islamic world.

It looks intimidating to attackers below — Wrong. Decoration was a side effect. Solid walls would actually look more imposing — the zigzag exists because of the cover-and-fire geometry, not aesthetics.

It lets rainwater drain off the wall top — Wrong. Wall tops have separate drainage channels. The crenellations are too irregular to drain efficiently — they're shaped for archery, not water management.