Shortly, I will send my PCs into a castle ruin, and I plan on them spending some time there, possibly to refurbish the castle and claim it as their own. I want to make this castle a realistic place, but I am stumped for resources on the matter, especially floor plans.
What are good resources to look for this kind of thing? I tried searching the web for 'castle map' and 'castle plan', but these mostly yield single- ot two-floored plans, while I'm planning of a large keep.
The book Castle by David Macauley is my go-to, along with pulling up floor plans of whatever various real castles I find. The book goes into details about the construction -- which will be important for describing the state of decay accurately, as well as informing what the players will need to find/do to repair/rebuild it -- as well as describing in great detail many of the most minor details, everything from why the foot of a castle wall is wider than the top, to the design of arrow slits, crenelations, and murder holes, to what the castle bathrooms look like. It gives you more than enough information that you can design your own floor plan for a plausible castle that features everything a real castle would need, plus gives you the fundamental understanding necessary to extend it to what a castle in a world with monsters and magic (if applicable) would have in addition.
The book is primarily illustrations, but the text descriptions are more than ample to understand everything. And, it's not just the castle, but also the town that was built next to it (including the roads, the town wall, gatehouses, and even the outlying parts of the town that grew up outside the walls as it expanded), and even goes into why the location that was chosen was chosen.
For all it gives you, you can easily read the entire book a couple times over in a lazy afternoon, and because of the heavy reliance on the illustrations it's very simple to later flip through to the exact spot you need when you need to reference a particular detail again. If you're the lazy type, though, you can use the book as a very detailed reference to the exact castle described, as it gives you not only floor plans (plus map of the surrounding town and terrain), but also exacting details of almost every single aspect of the finished castle.
Also related is the same author's book Cathedral; while not directly relevant to this particular question, it's an excellent resource in exactly the same vein to further your world-building efforts by including impressive, realistic cathedrals that are far more than the simple "temple with a big room where worshippers pray" -- I've used both books to accurately describe partially-constructed/partially-deconstructed buildings of these types. He's also got a much wider range of these books than I ever knew, too, many of which would be very useful to your world-building efforts.
Well, most actual medieval castles were pretty small, so you're finding reasonable stuff. For very large floor plans you need to go much later in time. You can find full floor plans of Neuschwanstein and other larger, later castles (Castle Loma, Castle Peles) on Randwulf's site. Even this huge, 1800's castle is smaller than a lot of fantasy castle floor plans I've seen.
Search for real castles, find a few you like, and then search for their floor plans specifically.
While searching for castles, you would do well to use search terms like "fortress" and "palace" depending on the kind of thing you want. Also add location/culture descriptors like "carolina fortress" or "moorish castle" or "aztec palace." (Hint: also try related terms like "monastery.") Do both regular web searches and image searches. Your searches will give you additional terms you can use to change or narrow your future searches, like "star fort." It'll probably take several passes to track down the kind of building you've got in mind.
Then look for maps of the place! Sometimes just Googling the name of the place and "floor plan," "map," or "blueprint" is sufficient. For example, if I want the Neuschwanstein Castle (which I found by looking at Wikipedia articles listing the inspirations for various Disney castles), "neuschwanstein castle blueprints" gives me plenty to work with!
Other buildings may be a little more reticent about giving up their secrets. Look for tourist brochures and hunt through Wikipedia citation links for more resources. If there's a place you've really got your heart set on but can't find good maps online, a surprising number of places will send complementary literature if you send a physical letter requesting it.
While there are many resources that will tell you about the features of castles, such at the Wikipedia Castle article, you should remember what a castle is for and how they were designed.
Castles are (mostly - Neuschwanstein is an example of an exception, as it was a vanity/fantasy castle created by an obsessed king) primarily defensive structures. They are designed to be force multipliers that enable a relatively small number of soldiers to defend a territory against a large number of enemies. The design of a castle depends on the technology at the time at which it was built or last modified, the threat(s) it is designed to defend against, and the location selected for its construction.
Since a castle is a defensive structure, it makes sense to locate it on the most defensible bit of land available.
Where the landscape is flat, castles often occupy any reasonably high rocky outcrop, or a low area that can be flooded to form a moat. Given sufficient technical advancement, the most common types of castle for these areas were concentric in design, where a defender would first have to cross the moat or a ditch, then pass an outer wall or gate, an inner wall or gate and only then would they be able to assault the keep, which was frequently accessible only from an upper floor to make things more difficult.
In more vertiginous terrain, castles often occupy high spots which were often rocky. Where the means of approach was limited, the defenses were oriented so that there was at least one wall between the keep and potential attackers in all but the most ludicrous directions (i.e. up the sheer cliff), often two or even more. Moats were rare in such situations, but a roadway that crossed a chasm via a drawbridge could be useful, as the chasm would have to be crossed.
Even in a keep, staircases were often narrow and spiraled clockwise going up so that a right-handed swordsman would have their sword hand against the central column while a defender higher in the keep would have freer use of their weapons. Narrow staircases limited attacks to the width of a single man. Doorways were often small to force single man attacks, and were occasionally very low so an attacker would have to bend double to get through, making them very vulnerable.
When planning a castle, one must consider what it was used for. If it was a small castle that was simply defending an area of land, it will be relatively simple, with storage areas, armories, smithies, stables, kennels, mews, kitchens, dining areas, sleeping areas for the defenders, and sanitary facilities (such as they were). Don't neglect the water supply - defense of a castle can end within days if this is compromised. The defenses may be as simple as a wooden palisade or as complex as stone walls with machicolations, hoardings, turrets, embrasures, etcetera. If the castle was the seat of a lord, there may be additional areas such as the lord's family's private rooms, art galleries, libraries, gardens, and any of a number of other non-military structures reflecting the ruling family's interests. Where religion is important, places of worship are frequently included where possible, their size and features reflecting the relative importance of religion in the defenders daily lives.
One must also consider the castle's history. Was it built as a single project and never modified thereafter, or was it built then expanded and/or modified on one or more subsequent occasions? This will affect its design, as a single-build castle will usually be more logically laid out (related functions placed near one-another) than one that was built and re-built as the dictates of necessity required.
Considering that this is rpg.stackexchange and not history.stackexchange, defending against enemies other than humans with purely medieval technology must be considered.
With the advent of gunpowder, walls by necessity became immensely thick and filled with earth to absorb direct-fire shot, and many defensive rooms were deep underground to protect against plunging fire.
Where magic is in regular use, a castle must incorporate defenses against magic. This is too dependent on the particular magic in use to elaborate on further here unless the question is elaborated upon with mention of a specific magic system or RPG.
If a castle must defend against flying creatures or land-based attackers riding flying mounts or vehicles, this would necessitate other changes. In one AD&D campaign I ran where the enemy rode wyverns and the defenders rode pegasi (with a tighter turning radius than pegasi), the open spaces between the castle's walls was draped in heavy spiked metal chain netting, and the sally ports for the defender's pegasi-riding knights were accessible only through approaches that required very sharp maneuvering (for a mounted pegasus), and included defenses such as heavy yard-long spikes in the turning areas and on the ground to catch a flying creature that could not maneuver as sharply yet still had to maintain a certain forward speed to remain airborne.
Given modern technology, almost no defense other than immense thicknesses of dirt and rock and well-concealed entrances can protect a fortress. Mobility is now a far better defense - consider that the president of the US has aircraft as a location of last defense rather than a bunker. If the location of a bunker is known, it can be destroyed - during the cold war, the Russians had a regiment of missiles with high-yield nuclear warheads specifically tasked to reducing Cheyenne Mountain to rubble.
Other features that I have included in fantasy RPG defenses is roadways that fold up against the walls to either side (which are spiked so that attackers on the road when the road is folded up will be punctured as well as crushed) to either side to reveal a series of pits filled with spikes, razor wire and flammable oil. You only need weights and architecture too, magic is optional. I've had metal-clad walls (with vertical triangular grooves of irregular width to prevent the use of suction devices in climbing them) that have included a drainable moat between then that means that not only are the walls slick metal, but covered in slimy algae. Don't neglect the possibilities of a few huge stone spheres ready to roll down that sloping u-shaped roadway between two walls (fill in those corners that attackers can hide in!)
If some of the defenses are underground, truly terrifying defenses can be set up with sufficient time and resources. Imagine long narrow switchback tunnels filled with arrow slits and murder holes, or the possibility of thousand-ton stone blocks that can be dropped on attackers. Tunnels can be flooded with water or more flammable - or simply hot - substances. Instead of a drawbridge or a wooden gate, move a plug of stone tens of yards thick on a layer of trapped water.
Finally, given that the castle in the question is ruined and has been abandoned (by its builders), there must be a reason why. Did its defenses prove inadequate? Did it not have sufficient supply of food or water? Was its damage too expensive for its former owners to repair? Had it simply become irrelevant or outdated? Is it still occupied by some being(s) that can't maintain the castle but are still capable of driving off its original defenders?
I have not run a campaign in 25 years now, but when I did I found Joseph and Frances Gies' books "Life in a Medieval Castle", "Life in a Medieval Village" and "Life in a Medieval City" good source books for designing realistic campaign settings.
In the back of the AD&D DM Guide there is a set of castle generation tables that can provide some pretty good constructions
Books on real-world castles are a great resource but real-world castle designers didn't have to worry about teleportation, disintegration, invisibility, flight, non-corporal beings, the Astral Plane, magical food/water generation, etc.
The Castle Guide is a sourcebook (for AD&D2) that discusses castle design in a magical world. I read this book when it came out and enjoyed it a lot. I think it is still a good reference for both D&D current versions and other swords & sorcery games. I don't, however, remember it having actual floor plans.
