Hi!
All castles we make are based on the book canon if applicable, but entirely made up by us. You won't find any blueprints that we went off of, but you should look at our project applications. Each castle has to be applied for by laying out plans for both the castle and the surrounding lands. They don't necessarily include blueprints, and certainly none that are closely reflected by the actual builds, but they will give you a good idea of our thought processes, and sometimes come with cute models of towns or keeps.
Some examples:
High Hermitage
Uffering
Fairmarket
If you're interested in Winterfell, here's our
fact collection which includes a few artists' renditions of the castle.
And of course there's also the
Livemap. Checking out our castles from a
birds eye perspective may help you figure out layouts. The
topography map can be of help, too.