Glacis - the gentle slope downhill away from old forts intended to simply aim
of musket marksmen in the fort - they would not have to be concerned about
elevation of aim.

glacis
n.,
pl. cis5Fr < OFr glacier, to slip < glace, ice < VL glacia: see GLACIER6
1 a gradual slope
2 an embankment sloping gradually up to a fortification, so as to expose attackers to defending gunfire