European castles have stood for long periods of time and they differ in design to the castles made during the medieval period.
Most of the castles were fortresses, thick-walled behemoths, usually without the luxuries of plastered walls, finely attired spires, and roofing patterns.
The most beautiful European castles are from the baroque times during the 1600’s and 1700’s.
Later periods would erect truly sumptuous buildings in the middle of large cities that would resemble nothing what most people think as a castle.
These are greater than what you see in Game of Thrones but have no fortress character at all.
The Red Keep
This is more or less realistic if you consider it to be a well defended palace than a traditional castle, since it’s taller and more delicate compared to a traditional castle.
The outer walls look like they were added at a later date showing that the Keep expanded through the ages. This gives it more history and makes it more believable.
This is extremely realistic except for where it is located.
The stucture is sprawling, not as tall, solid, or well fortified.
However, the fact that it is based in the North and the roofs of the towers are flat makes it unrealistic.
This would cause the castle to cave in due to the weight of the snow that would pile up during the years long winter that happens from time to time.
Typical castles in such locations have very steep roofs.
The towers on the right would’ve collapsed long ago due to erosion.
There is a slim chance this castle would bare the test of time.
Again, nope. This is ridiculous. It would be ridiculous even if the mountain it’s built on had not been carved inside to create those pillars that support the castle.
Apart from the above, Casterly Rock is said to have been built into a mountain.
While not impossible, it is improbable.
There is no reason to build a castle into a mountain when it’s infinitely easier to build on top of it.
To conclude, dragonstone might be the most realistic of them all. Although it is far too close to the coast to not be affected by soil erosion over time.