The concept is as old as warfare — or snowball fights.
When defending a building, a fort, or a castle– create a way to shoot out without unnecessary exposure to the attackers.
Hence: snow forts. At least that was the seasonal result when and where I was a child. Build up a wall of snow. Throw at the opponent. Duck. Quickly pack a new projectile.
You still see the concept on the evening news with reports from war zones. Defenders use windows, or shell holes, or create small spaces where they can fire their weapons out while keeping a barrier between them and incoming fire.
Today’s photo is an example from several centuries ago. Picture the scene. You are in the castle, standing on an earthen embankment inside the wall. The enemy is massing at the foot of the hill. Wait. Wait. Patience until they are within range. Don’t waste ammunition.