Here is some work in progress in single player.
The first screenshot is taken from the top of the natural archway that is the entrance to the valley.
and this is the only finished room, the dining room.
The oldest building is the watch tower on the top right in the first picture. A day I was exploring and when night fell I was still outside. So I got to the top of a mountain ( the cliff you see is natural. on the other side, however, the slope is much less extreme and perfectly walkable) and build a small house. having time and cobble to waste, I also built a tower, so that I could watch the surrounding area safely.
Then, I built the small wall you see protecting the castle. It was a small flat strip in the mountain, easily walled and safe. Also, it had a rather interesting feature: there was a big, tall and almost empty natural cuboid in the mountain. Then started the idea of having a castle: how could I not turn that natural shape into a tower? and so, I started replacing rock with cobble. Some people who watched my livestream might have seen it while still all natural, or almost all.
The result is the 3 floor structure you see ( no windows yet).
Then, I decided that a castle needed to be bigger, and so started carving more rooms in the mountain, since there was much space. The first 2 rooms are already built: a dining room, shown in the picture, and a still empty personal library of the resident noble.
There is, however, much work to be done yet. in the same direction of the dining room, I will build a church. in the right of the first picture, in the mountain below the tower, there will be military quarters. Stairs are still missing in the main tower. Rock needs to be removed. But I am happy with the results