Glacis
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Basic Information

A glacis is a defensive structure built of rammed earth that is positioned in front of the walls of a post-gunpowder fortification to protect them from cannon fire. The glacis traditionally takes the form of a smooth ramp facing the enemy, backed by a vertical retaining wall at a significant distance from the curtain wall it protects - there may even be a moat at the bottom of the gap. Since the glacis must be as tall as the curtain wall to be effective (since anything higher can still be shot at) the gap between glacis and wall is necessary to avoid the enemy simply walking up the ramped face and over the wall. The ramp itself has two aims - the first is to either adsorb incoming fire or skip it over the wall (accomplished by a slope of unfaced earth) and the other is to serve as a killing ground for assaulting infantry (since it is smooth and open, with no cover or dead spots and a clear field of fire for the defenders on the wall and eliminates the natural dead zone generated by the wall itself). The glacis generally rises to rampart level of the wall it defends, leaving only the actual battlement exposed so that the defenders can fire over it. Particularly assiduous designers may well place an additional moat (often a wet one) at the bottom of the outer face to make it harder for the enemy to get onto it in the first place.

Once explosive shells become widespread the glacis becomes less useful, since the earth can be relatively swiftly blasted out of the way - and indirect fire makes walls less and less useful anyway.

The precursor to the glacis generally consisted of reinforcing one or both sides of the wall with piles of earth … this had the obvious problem of reducing the effective height of the wall when applied to the outside, so the seperation gap was invented fairly swiftly thereafter.

Sources

Bibliography
1. full source reference

Game and Story Use

  • In peacetime, sheep and/or goats can be usefully grazed on a glacis … apart from anything else they keep the face from becoming overgrown.
  • These were often added retroactively to medieval fortifications once cannon became widespread.
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