How did the battlements along the top of a castle wall work?
A continuous solid parapet ran the whole length with no openings at all
Raised sections alternated with gaps, giving cover and shooting openings✓Bonne réponse
The whole top was left open, with no raised sections anywhere
A projecting gallery hung out beyond the wall face with holes in its floor
Explication
The alternation is the whole point: a defender stepped behind a solid upstand for protection, leaned into the neighbouring gap to shoot, and stepped back under cover again. The projecting gallery fails because that is a separate fitting built out over the base of the wall so that defenders could drop things straight down, whereas battlements sit flush along the wall head.