半月堡

Also known as Demi-lune

thumb|Two ravelins (top left of image) in Goes, The Netherlands thumb|Ravelin in the Dutch town of Bergen op Zoom thumb|300px|Ravelin protecting the entrance of Fort McHenry, [[Baltimore, Maryland]] thumb|The Moers fortifications, designed by [[Simon Stevin, where ravelins appear as triangular shapes surrounded by water, with wall (shown in dark green) facing outwards with no wall on the inner side.]]