Before there was Curse of Strahd (CoS), possibly the most famous 5e adventure, there was the 3rd edition adventure Expedition to Castle Ravenloft. Instead of running CoS, I sometimes run Expedition. It's fun for players who have never experienced Ravenloft before, and it's got a few surprises for players already familiar with Curse of Strahd. I'm reviewing the 3rd edition book, chapter by chapter. There will be minor spoilers in these posts, so this is primarily intended for Dungeon Masters.
Chapter 4 is the castle.
I hate to start with the weak points, but the maps in this chapter are pretty bad. As much as I love the layout of the Expedition to Castle Ravenloft book, I have to confess that I find isometric maps unhelpful and confusing. I'm not sure why they're isometric, aside from tradition (and even then, why were they ever isometric?) but it only makes it harder to reproduce the floors as a 2d battle map.
The castle is also big, with over 80 numbered locations. For some reason, all locations, regardless of which floor they're on, are prefixed with K (K1, K12, K78, and so on). The castle would be so much easier to navigate if only each floor had been given its own letter prefix: K for the ground floor (because K was the next letter after the previous chapter), L for the first floor, M for the second, N and P and R and S for towers and rooftops and so on.
Those are the two aspects of this chapter I don't like. The rest of it is flawless.
The thing about Castle Ravenloft in this adventure is that it's Strahd's lair, and so he could be around any corner. Madam Eva's fortunes foretell exactly where Strahd can be found within the castle, but she doesn't exactly give the player characters a location and encounter number. She gives clues that are likely meaningless to anyone who's never been to Castle Ravenloft, and pprobably pretty vague to even people who have. So Strahd is pretty much equally likely to be in one given room as any other, and if it's not Strahd waiting for the PCs, it's probably something nearly as dangerous. This isn't just a dungeon crawl, it's a manhunt.
The intriguing thing about Castle Ravenloft is that it was once a fortress of good. By all accounts, Barov and Ravenovia, the parents of Strahd, were benevolent rulers. Accordingly, there are hints of that still, however decayed and faded, in the castle today. That adds a little sadness to the castle, but it's only one of the many small stories embedded within the walls of Strahd's home.
As players move through the castle, histories emerge. Some stories have little to do with Strahd directly, and others can have a huge impact on him. It's difficult to tell one from the other, of course, and even more difficult to know who's safe to ally with.
It might seem strange that there are inhabitants of Strahd's home who don't necessarily interact with Strahd, but the castle is huge and ancient, and I think the implication is that Strahd doesn't generally go on leisurely afternoon strolls through the halls of his home. I think he keeps to the rooms in which the Fortunes of Ravenloft list as potential encounters, and otherwise the castle is essentially abandoned. There's an open invitation for fiendish squatters, poltergeist, golems, troll stalkers, devils, and just plain nasty humans.
Strahd's home isn't regal, but I don't get the sense that he's aware of its decay. I like to imagine that he's too far gone to notice the tangible gloom around him. He's an evil guy, whether he's depressed or not, but the state of Castle Ravenloft, I think, is a reflection of Strahd's mental state: dismal, conniving, desperate, aggressive, and supremely unpleasant.
Like the rest of Barovia, the castle's stories are perfect for all kinds of players. Some of the encounters are immediately aggressive, but there's a lot of opportunities for conversation, negotiation, and role play. Many story seeds that were planted in other regions of Barovia come to fruition here in the castle, so there are many quests that can get resolved here. The ones I find most intriguing, myself, are the ones that actually don't matter. I won't spoil any of them, but there are at least two people I can think of who you meet in the castle after only hearing of them previously. Temper those, though, with several depraved individuals you'll meet along the way, and you have an unsettling and sometimes terrifying experience.
There are a lot of familiar monsters, straight out of the Monster Manual, but there are also a fair few unique monsters as well. If you're running this for 5e, Van Richten's Guide to Ravenloft is a required supplement. Although it doesn't have all the monsters from this book, it does have nicely horror-themed monsters that you can either re-name or re-skin and get close enough as needed.
If you're running this as a Pathfinder adventure, everything works with few to no changes.
The castle is the final chapter of the book, and there's no "Concluding the adventure" section at the end, as there often is in a linear adventure. This is Ravenloft. It's a "sandbox," a non-linear adventure that expects players to roam the countryside, gathering clues, and only in the end approaching the lair of the evil overlord. However, both this book and Curse of Strahd emphasize that Strahd shouldn't be saved for the very end. He's the main attraction, and if he's naught but a name until the final few game sessions, you haven't really experienced Strahd's dark domain.
To that end, I like to have Strahd invite the players to dinner pretty early on in the adventure. I like to get the players into the castle, so they can see the state it's in, and even explore it a little. Strahd always has a Secondary Goal, after all, and it ultimately it was him who beckoned the player characters to Ravenloft. Meeting them in person under the auspices of hospitality seems not only reasonable but also pretty classic. Heck, you might even have him invite them to stay the night, when rain starts pouring down and the news arrives that the bridge on the road back to Barovia has been flooded.
Whatever the excuse, I don't like the castle as a single serve location. I want it to be a place that players actually get to explore and experience, beyond going in surgically, at high levels, to extract certain items on their way to slaughter Strahd once and for all.
The final section of the book is the appendix, and I've actually already covered the bulk of it. The appendix contains lore and mechanics for Knights of the Raven and for Lightbringers, a new spell, and some magic items.
Obviously I'm a fan of Expedition to Ravenloft. Whether you run it for Pathfinder or 5e, if you're a fan of Curse of Strahd, then you're sure to enjoy this canny "alternate version."