The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 5: Forts and Artillery.
Francis Trevelyan Miller, Ed.

McClellan's army was delayed a month before the Confederates evacuated.

The preliminary reconnaissances by the Federal engineers persuaded McClellan that a regular siege of Yorktown was necessary, and accordingly strong works were erected opposite those of the Confederates. Emplacements for heavy guns and parapets to protect them were pushed to completion. Regular siege-works, consisting of “parallels” and “approaches,” were projected. The Confederates held the position until the last moment, and just as fire was about to be opened on them they abandoned the lines. By that time the works of defense had assumed almost the proportions of a fortress. Enormous labor was required to effect this, and, correspondingly, the labors of the besiegers were great. The low-lying ground of the Peninsula was under water part of the time from the tremendous rains, and the heavy guns of both armies sank into the mud, and it required tremendous exertions to extricate them. Yet, without fighting, the purpose of the Confederates was attained — that of delay; and, while many guns had to be abandoned, the expense was compensated for by the increased preparations of the main Confederate army.

But, notwithstanding the lessons in fortification given both combatants by these operations, the individual soldier did not appreciate, to any great extent, his own responsibility in the matter of entrenchments, since these Yorktown works were on a large scale and used by the entire masses of men of the hostile armies. It was in the campaign to follow that the important instruction in the art was to come.

The progress of the Federals was energetically disputed by inferior numbers in field-works at Williamsburg, which was not so solidly fortified as Yorktown. A large Fort with six redoubts bar-red the road into the town, but, with the flanks not well protected, the position could be turned, and the Union troops did not wait to undertake a siege. At Mechanicsville, Gaines' Mill, Seven Pines, Malvern Hill, and Harrison's