Orillon

An orillon was an archaic structure used in permanent fortifications to protect bastion flanks against slanting or enfilading fire by prolonging bastion faces beyond the natural point of intersection between the lines of flanks and faces. Orillons were, basically, an attempt to hide bastion flanks behind bastion faces.

Construction of the Magistral Line of an Orillon and Recessed Flank Based on a Bastion Front of Fortification of 200 Yards on the Exterior Side of the Polygon with Flanks Perpendicular to the Lines of Defense.

1. Divide the flank AB into three equal parts. Line segment AD represents the gorge of the orillon.

2. Produce two lines from the flanked angle of the collateral bastion (C, not shown) to points D and B.

3. Extend each of these lines 10 yards to points F and G.

4. Produce a line from point F to point G. Take this line as the base of an equilateral triangle with its vertex at point H.

5. Describe an arc centered on point H through points F and G. This arc is the magistral line of the recessed (retired) flank.

6. Bisect line segment AD and produce a line perpendicular to the bastion face from point A. The point where these two lines intersect, point K, is taken as the center of an arc from A to D that forms the magistral line of the orillon.

7. Line DF is the magistral line of the reverse of the orillon. Line BG is the magistral line of the brisure of the curtain.

January, 2003