The general idea is that an original castle was built over at a later time, similar to European castles, so the original turrets were used for different purposes and built out as such. You should be able to see the original turret in most of the main towers. The barracks tower and armory were left mostly original except for the connecting suspended footbridge. The architectural styles of the later additions and castle village buildings is influenced by Northern European town architecture, Scandinavian timber churches, and, to a small extent, the "Skyrim" series of games.
The big bridge up front to the right I imagined as a medieval suspension bridge, so it has components based on fortifications, Gothic arches, a bit of fantasy, and, of course, some kind of suspended timber structure in the middle, perhaps supported by giant intertwined ropes or some such.
The island itself is very steep and has a number of small tunnels, caves, possible secret entries, shacks for the fishing boats, and just a little bit of sparse vegetation and a few small trees.
The towers and other buildings are labeled if you look carefully, as per the requirements of the story.
Technical description: Done freehand pencil on paper, this is a completely original design for a castle. Pencils used were F, 2H, and 4H. Then I scan, clean it up, and gently tweak the color. Finally, I label it.
If anybody else out there wants a personal castle design, either for a novel or book, or as an idea for a game, send me a note and we can discuss the complexity of the castle you need, extra drawings like ground plans or more detailed drawings of individual buildings, and anything else, and then I'll try to come up with a reasonable price. I am getting booked up into late December at this point with commissions, design, and construction projects, so I probably wouldn't try another of these for a number of weeks, but we can put it on the schedule somewhere I'm sure.
Note: File changed out and updated on Dec 16, 2012, new file has the ground plan.