Dunharrow

By the time of the War of the Ring, this was the main gathering area for national musters of Rohirrim.

Apparently, the area was fortified. But I have no information on what form these took in the books. The area was pretty well defended as it was, so little would be needed to fully secure it. Though the first thing that jumps out is the village. Normally, I would think that the villagers would be better suited by locating their homes in the Amphitheater. But this area was justifiably reserved for the military camp site. Plus, access up that stair would have pointlessly restricted trade. Meanwhile, locate the villages down in the valley and more and better farmland could be found plus trade links via the river.   Now remember the ethnic and cultural (and probable trade) connections Rohan has with the Northmen? This suggests to me that the two villages here would be combined into one village using the Motte and Bailey style settlement I describe for the Northmen. The villagers would most likely be part of the government apparatus of Rohan, given their proximity to both Edoras and the main muster site for Rohan. Which means the people would be the most settled of all of Rohan outside Edoras. So this Motte and Bailey idea would make much more sense here than it would elsewhere in Rohan.   At the top and bottom of the stairs would likely be the center point of the defenses. This would likely be a wood gate house at each end. Small wood towers would likely be located at the top to fire down on the enemy as they march up. I would also put some form of feature at the top to allow for dirt or other debris to be dropped on the path to allow it to be permanently blocked in emergencies. It might be wise to install some form of temporary barricades on each leg so that soldiers can throw them up during attacks. But this is much more optional. Earthworks would probably be overkill at this point. But some form of earthworks with a more developed gatehouse would make sense at the mouth of the valley. But again, not much would be needed. Most of the time, civilian traders would be the main traffic - which is unwise to impede without reason. Plus the river would make too much of a wall pointless.

Dwimorberg (the Haunted Mountain) was their base of operations. It was located in the White Mountains. It was described as "brooding" and black. Access was through Rohan's camp at Dunharrow. Directly south of Edoras was a deep north-south valley called Harrowdale. This had the tiny villages of Upbourn and Underharrow, which we can assume are the homes of some farmers and other craftsmen needed to supply the Rohirrim when it musters. It was formed by the Snowbourn river, which flowed past Edoras. and joined many of the important rivers in the area.   Starkhorn Mountain was the source of this river and its snow capped peak could be seen from Edoras. On it was a cliff forming the natural amphitheater of Dunharrow. Access came via the Stair of the Hold - a tight and narrow path forming a series of switchbacks. It rose hundreds of feet like this, giving it the stair like appearance and making attack quite difficult. A small fir forest was located in the back, with a double staircase. A dark door was at the top, allowing entrance into the mountain and the Paths of the Dead. The paths were dark and radiated fear. In no small part this fear came from the constant whispers of voices in an unknown language. It was here that the Oathbreakers resided prior to being called by Aragorn. In the movie version, this is where they are summoned.   In the books, they are called on the far side. Here, Isildur placed a black, spherical stone with a six foot diameter on the Hill of Erech. This stone was rescued from Numenor as the Island sank and was used by Isildur as a symbol of his heritage. It was here that the local tribes swore their oath.

The area was originally settled by the first Wild Men and the future "Oathbreakers". At that time it was a sacred space. Later, Rohan turned it into a marshaling area and refuge in times of crisis. In fact, one of the groups of troops that managed to outlast the Wild Men occupation of Rohan hid here. They then helped the survivors of Helm's Deep retake the capital. It was through here that Aragorn, Legolas, Gimli and the Grey Company had to pass in order to make contact with the Army of the Dead. As they were doing this, Theoden was marshaling the Rohirrim outside before marching to Minas Tirith.