Line of Redoubts and Redans. Sabine Pass, Texas.

During the middle period of the nineteenth century military engineers were at odds as to whether it was appropriate to fortify an extended position with field works closed at the gorge. In the event that a detached work in line of entrenchments was captured a work open at the gorge would be more difficult for the attacking force to hold against a counter attack and its interior would be exposed to artillery and small arms fire from other works to its rear. An enclosed work with a parapet covering its rear, on the other hand would be much easier for an attacker to defend and use to continue his attack against the line, it would certainly be much more difficult for counter attacking troops to re-take. But, if well defended by a determined garrison an enclosed work could not be turned by an attack directed against its gorge and its defenders could persist in their defense even after attacking troops filled the intervals between works. Both solutions were used during the American Civil War and both pretty much held to expected consequences.

This redoubt and redan line with intervals, taken from D. H. Mahan's Treatise on Field Fortification, combined a front line of square redoubts with a second line of redans.

Draw a right line of any length that will serve as the front boundary line. Mark points of division along the line at 250 yard intervals. These points of division mark the locations of the forward salient angles of the first line redoubts.

Construct square redoubts at each point of division along the front boundary line. Each side of each redoubt should be made at least 60 yards long. There are several ways to accomplish this task, in this case perpendiculars were dropped from each point of division to create two right angles on the interior side of the boundary line. Both right angles were bisected to establish the positions of the forward two faces of each first line redoubt; the bisecting lines were then extended a distance of 60 yards to complete the forward faces. The two rear faces of each redoubt were constructed as the final two sides of a square.

To obtain the location and orientation of the second line of redans begin by extending the lines of the redoubts two front faces to their point of intersection in rear of the line of redoubts.

Construct the faces of the redans by marking points 30 yards on the redoubt face lines from their point of intersection and constructing perpendiculars toward the engaged side of the line of works and carrying them to their point of intersection. This insures that the faces of the redans are perpendicular to the faces of the first line redoubts that they are intended to support.