Started for the collab but some
Harlech castle, the perfect castle built by James of St George, designer and constructor of Edward I’s great castles.
Ground plans are not hard to find but much harder to get elevations.
I started from a ground plan and blocked out the walls and towers, some internal ones very basically.
Gradually I will make the internal parts, spiral staircases, and windows etc.
A good find was a photogrammetry model of the castle. Free download. However, it is terrible mesh, as expected. Aerial photos low resolution considering the area covered means it has low info to use. However, it fitted the block out built on the ground plan really well in respect of size and shape. Photogrammetry is accurate in those respects. So I have used it as a reasonable guide to the heights, making allowances for the slighting damages from after the Civil War.
(Small note, the photogrammetry model was in a .ply file. Which Blender can import. But, as I found out, there are two types of them with no real distinction bar reading the file in a text editor. So of course it was the version that Blender can not read the texture rather than the one it can. Not a big deal as it is of no real use for my purposes.)
(Crenelations added later.)
Now with the photogrammetry showing.
The greatest thing about this is I have a great basis for the real ground! That would have been a nightmare to work out. Many modern houses to get rid off but it will be so close to the real shape! 
Wireframe might show the fit better.
After the block out I started the crenelations. Using arrays and fitted to a curve for the towers.
Corbels under the crenellations on the Gatehouse rear towers too. Sorting the seams and UVs of the crenellations to use for the collab entry was quite a task.
The texture is not great, but I might be able to lighten the repeating dark patch. Basic rocks seem pretty reasonable short of something based on the real thing.
As you can see I started cutting out the entrance passageway.
Soo much more to do. lol.