Castles of the Inner Sea is a 64-page entry in the Pathfinder Campaign Setting line. It provides an in-depth overview of six different castles in Golarion. After a short introduction and terminology section, each ten-page section for each castle is divided into the following topics: History, Description (keyed to a full-page schematic of the castle's interior), Denizens, Threats (including a new description and stat-block for an NPC or monster), and then a fully-mapped and statted area of the castle suitable for adventuring. Each section also includes helpful sidebar adventure hooks to get the PCs there and some example encounters they might find there or in the area. Before getting to specifics, special mention should be made of the art and layout of the book: it's beautiful. The drawings of each castle from the outside perspective are gorgeous and evocative, while the interior maps are detailed and useful. Scattered throughout the book are drawings of key NPCs. The interior artists and cartographer deserve praise for their work on this one. Now, on to the castles represented:
1. Castle Everstand, a fortress on the border of Lastwall that helps to keep the orc hordes of Belkzen at bay. This is a castle that would make an excellent "home base" for low-level PCs, as there's a small village just outside the city gates to serve their needs, but enough CR-appropriate threats nearby (recon missions in orc territory, for example) to keep adventurers busy. I liked how, although this is a "good" castle, tension is developed by explaining how the leaders of Castle Everstand are barely resisting the temptation to change from a purely defensive mindset and instead launch some sort of crusade to take the fight to the orcs--there's some good storyline potential there. Full stats are provided for a CR3 "Lastwall Border Scout" (perhaps someone who could escort the PCs on their first couple of missions). The adventuring locale, suitable for four 1st-level PCs, is the Everstand Crypts, a long-abandoned, partially flooded level below the castle full of vermin and some undead. Reading through the section on Castle Everstand gave me a "Keep on the Borderlands" vibe, and I think it would work well as a starting area for players brand new to RPGs or Pathfinder.
2. Castle Kronquist, a gothic citadel in Ustalav that is home to incredibly powerful threats of the undead and vampiric nature. This location is about the exact opposite of Castle Everstand, and is designed for high-level (13th to 16th level) PCs. The history of Castle Kronquist is really interesting and well-written: the fortress was placed under siege during the Shining Crusade as it was home to one of the Whispering Tyrant's most powerful generals, a vampire lord named Malyas. But the siege of Castle Kronquist was lifted after nearly a year because the building would always reconstruct itself overnight! The knights of the Shining Crusade kept close watch on the citadel for years before finally deciding that Malyas must have somehow escaped, and now the place is long-abandoned. The PCs might come to Castle Kronquist to discover the truth of Lord Malyas's whereabouts, but before they do they're likely to encounter Cvotgar Haas, a former vampire hunter who has been turned! Full stats of Haas are provided, along with details on the area of the castle where he resides (designed for 4 15th-level PCs). This is clearly not the sort of place normal PCs want to make their home, but it could make the basis for a good story arc for high-level characters.
3. Citadel Vraid, forbidding headquarters of a Hellknight division, the Order of the Nail. Located in the Mindspin Mountains west of Korvosa, this castle is actually three fortifications connected to one another only by a very defensible bridge. The Hellknights (Lawful Evil agents of a nation that has made pacts with demons) are a very popular part of Golarion; I've never had much to do with them, but I could see why Citadel Vraid would be an interesting location for PCs to visit. Full stats are provided for a typical Signifier of the Nail (a Hellknight with sorceror levels) and for Mistress of Blades Maidrayne Vox, a centaur Hellknight general. The adventuring locale provided is The Hellfast, a labyrinth under Citadel Vraid wherein the leaders of the Order of the Nail conduct their rituals of summoning and binding. Come to think of it, this might be a good location for a storyline involving deception and infiltration, as the PCs could be tasked with retrieving/accomplishing something in the Hellfast without being discovered and bringing the whole castle down upon them! I would note that, for a castle intended for high-level use, more magical defenses should probably be detailed to keep the usual PC tricks from being employed (teleporting, passwall, gaseous form, etc.).
4. Highhelm, a dwarven metropolis high up in the Five Kings Mountains. Highhelm is one of the legendary Sky Citadels, and is said to contain great secrets and treasures of the dwarven people. From my reading, Highhelm really does seem more like a walled city than a "castle" per se, and although reasonably interesting, I'm not really sure it belongs in this book. Unlike the other castles, the Threats section for Highhelm doesn't list NPC stat blocks but instead provides a couple of new items: a powder ball (an explosive device) and a Sky Mine (a hot-air balloon that explodes when hit with a flaming arrow). The adventuring locale, suitable for four 4th-level PCs, is called Deepscar Keep, an abandoned fortification several hundred feet below Highhelm that has been reclaimed by some of the denizens of the Darklands. Overall, this was the least interesting entry in the book for me; but, I suppose, if you have a particular interest in dwarves or need a PC base for major exploration of the Darklands, it could be useful.
5. Icerift Castle, a stronghold in the forbidden arctic wastes of the Crown of the World in northern Mendev. Abandoned for a century, the backstory to this one is fascinating (if grisly), involving cannibalism and slaughter. Now, the castle is inhabited by a vicious tribe of Wikkawaks, which are apparently white-furred bugbears that live in arctic climes. The Wikkawaks are led by a CR 12 arctic druid named Ugmitok, and she receives full stats and a picture. Ugmitok's clever trick is to summon a dreaded wendigo with a loud blast of a horn, and then flee with her fellows to a barred shelter so that the wendigo attacks everyone else while the Wikkawaks are safe! Below the castle, the adventuring locale provided is The Ice Caves, a series of catacombs for four 10th-level PCs. The Ice Caves are where the Wikkiwaks live, and their lair is guarded by a couple of fun traps. I imagine it would take a really pressing adventure hook to get PCs to come to such a remote location, and I don't imagine them sticking around for long once they do (unless they like the cold and are fond of teleport spells).
6. Skyborne Keep, a fortress of storm and cloud giants that flies through the skies to raid trade routes and settlements below! I thought this was the most fun and original idea in the book, as it's easy to throw into almost any campaign (once the PCs are in the double-digits level-wise). The premise is that a group of Pathfinders uncovered an ancient but surprisingly well-kept castle on a remote mountaintop. The Pathfinders were discovered and attacked by a power-hungry storm giant named Lona Orames. Only one Pathfinder survived (a half-elf named Bastian Saloni), and he did so by making a deal with Lona: he revealed the secrets of Skyborne Keep and would help her learn how to fly it! Bastian now serves as Lona's trusted adviser, and uses his knowledge of precious magical treasures all over Golarion to help the residents of Skyborne Keep raid and pillage. I like the emphasis in the description that the leaders of Skyborne Keep are smart and patient, and thus avoid becoming predictable and falling into ambushes. A good part of the story involving Skyborne Keep could involve the PCs simply trying to figure out where it will strike next. Lona Orames receives a full stat-block (and I'd guess you haven't seen many storm giant monks before!), while a (rather dangerous) adventuring locale is the lower levels of Skyborne Keep designed for four 17th-level PCs). The only thing missing is any hint of who built the castle to begin with.
Castles of the Inner Sea is one of those books that may not be what buyers are expecting. At least half of the book (Castle Kronquist, Icerift Castle, and Skyborne Keep) detail direct threats to PCs and are thus much more in the form of "dungeons" (broadly speaking) than they are castles that PCs could visit and make use of. One of the locations (Highhelm) is really more of a walled city than a castle. That being said, this is a really great book, and in some ways would be better value for its money than Dungeons of Golarion because it's more detailed, more practical to use in a campaign, and much more GM-friendly. My advice would be to ignore the title, scan the summaries of the different locations discussed, and buy the book if you think one or more of them would be useful in your game. As I said above, the artwork and maps are excellent, and they could also serve as the location for custom threats.