I understand your point about Fort Donelson and Henry, but my central point in that post was that not every battle saw a conventional stand up fight, and earthworks saw use at Shiloh, Stones River, Corinth, many of the Seven Days battles (like Seven Pines, Gaines' Mill, and Glendale) battles of the Vicksburg Campaign such as Chickasaw Bayou, Port Gibson, Raymond, Jackson, Champion Hill. Many of these battles did not see such extensive works as what were built, as I said before, in the Overland Campaign or Atlanta Campaign for instance, but earthworks and breastworks were built on the battlefield and were used in battles that were relatively early in the war.Earthworks like Fort Donelson, Fort Henry and other forts or batteries don't count. They were planned that way to make a place more defensible by a smaller force. We were talking about tactics and if forts were to be included, we should be discussing Vauban and siege warfare. The Siege of Yorktown, Vicksburg and Battery Wagner are great examples of siege craft (though neither Yorktown nor Wagner per se were sieges).
It's expedient fieldworks that I was talking about. Read the early accounts by soldiers and they thought it was unmanly to hide behind a tree during an open battle. By 1864, tree hugging and earth hugging were common and it was though foolhardy to charge an opponent who was behind earthworks. Fredericksburg taught the Army of the Potomac that and Gettysburg did the same for the Confederates. The Union Army of 1863 thought that Meade's planned attack at Mine Run would be another disaster ala Fredericksburg. They were overjoyed when Gouvernor Warren called it off. We see both sides digging in at The Wilderness, Spotsylvania Court House, the North Anna all the way until the breakthrough at Petersburg (at Five Forks). BTW, Professor Hess discusses the subject in his books. He also addresses the fallacy that the rifle musket changed warfare all that much in another of his works.
I understand that as the war went on, larger and more extensive earthworks were used in a more similar way to what was seen in WWI. Everytime an army would stop they would dig in and when the other army caught up with them, they would dig in. This wasn't seen early in the war except for siege warfare, but my point is that earthworks were present from 1861 - 1863 and in some cases in the battles I listed above, played an instrumental role. As for "Tree hugging and earth hugging," that was seen early in the war as well. Even at First Manassas, regiments were ordered to 'lay down' to stay under the fire and on numerous occasions the reverse slope of a hill, a ravine, railroad cut, or sunken road were used for cover.