Setting is an important part of any roleplaying campaign. Different people may like different levels of detail, but there is always, at the very least, an implied setting, a place where everything is happening—even if it's just the dungeon the characters are currently adventuring in. Setting grounds the characters in a certain reality. It creates certain expectations and lays down certain limits. It helps define where the characters come from, who they are, where they're going, and what they want. Generally, a well-defined and memorable setting paves the way for memorable adventures.
Adventures can use settings in different ways. Sometimes, the setting is little more than window-dressing, with little effect on the adventure itself. It might also be a relatively generic setting, one that can be easily modified or inserted into another more detailed setting. It's important that there be adventures like this. Gamemasters often need adventures that they can grab on a moment's notice and use with little to no adjustment. These adventures need to fit in regardless of the setting any particular GM needs.
But sometimes, adventures are tied much more closely to their setting, to the point that the adventure really couldn't happen anywhere else—not without some major changes, at any rate. Obviously, adventures like this will only work if the GM is using the particular setting, and they're not the sort a GM can just grab at a moment's notice. They need to be a planned part of the campaign. Yet, these adventures are often amongst the best and most memorable of adventures. That's not to say adventures with a more generic setting can't be great and memorable—just that those with a highly detailed setting enjoy a bit of an edge in the race.
Lords of Rust by Nicolas Logue, the second part of Iron Gods is an adventure where the setting is all important. It is a sandbox adventure in pretty much the truest sense of the term (something that is difficult to do in an adventure path). The player characters can pretty much proceed however they want and the setting is almost entirely what drives the action. A poorly detailed setting could break the whole adventure. But this adventure doesn't have a poorly detailed setting. Instead, it has one of the most memorable settings I've seen in a fantasy RPG adventure, and it makes for what will likely be an extremely memorable adventure for any group of players.
I should clarify what I mean by setting for this particular adventure. All Pathfinder Adventure Path volumes are set in the world of Golarion, so of course they all have a setting, and they pretty much all make a concerted effort to use their setting to full effect. But the various Pathfinder Campaign Setting supplements can only do so much, and often in a broader fashion than is needed for an adventure path or individual adventure. Each adventure has a much more localised setting and thus needs to do further development of its own.
Lords of Rust is set primarily in Scrapwall (apart from a brief opening section of the PCs travelling there), a town in Numeria. People who have read my review of Numeria, Land of Fallen Stars will know that, while I think there's a lot of good stuff in that supplement, it doesn't do a very good job of describing what it's like to live in Numeria. Lords of Rust goes a long way towards rectifying that—if just for the town of Scrapwall. In the pages of this adventure, Scrapwall becomes a living, breathing place, full of detailed individuals, all with their own conflicting goals and desires. What's particularly impressive is that it does this without the benefit of having a support article about Scrapwall later in the volume (and Scrapwall only gets a single paragraph of description in Numeria, Land of Fallen Stars). In fact, it does it better than many adventures that do have support articles later in the volume (Torch in Fires of Creation, for example).
The adventure opens with the adventurers setting out from Torch with the intention of tracking down where Meyanda (the villain of Fires of Creation) was sending the town's leached energy to. Now, I should point out that this adventure does require a group of PCs that are relatively self-motivating. If they aren't, GMs will have to do a little bit of extra work to get the PCs started and to keep them motivated throughout. The adventure does include a few possible hooks to get the PCs started (including some options to make sure they know the next destination is Scrapwall), but overall, the PCs don't have much reason at this point to continue the adventure path other than their own personal reasons. I don't think this is necessarily a bad thing, just something GMs need to be aware of in advance.
On their way to Scrapwall, the PCs make a brief stop at Aldronard's Grave, an old fort run by crusaders of Sarenrae. However, the fort has been attacked and taken over by bandits. The PCs defeat the bandits, rescue the prisoners and possibly also put to rest a ghost that has long haunted the fort. The main purpose of this brief section is to provide the PCs with some information about Scrapwall (NPCs in Torch suggest to them that they stop here to get information). As such, some groups may well choose to skip it (as there really isn't a whole lot of reason to go there), but if they do, it won't really impede their progress. They'll miss out on a bit of information and a few experience points, but they can find both those things easily enough in Scrapwall.
When the PCs reach Scrapwall... Well, from there, just about anything can happen. The adventure becomes entirely directed by their actions. Who they talk to, who they fight, who they ally with are all determined by the PCs and their actions. Because adventure paths have an overarching plot that needs to move forward, the more sandboxy AP adventures still tend to have a list of events that occur even while the PCs otherwise go about things in their own ways. Lords of Rust is unique (amongst adventure paths) in not having such a list (at least, I can't recall off-hand any others that do). There are no predetermined events at all.
What Lords of Rust does have is detail on the various gangs that operate in and run Scrapwall, including the titular Lords of Rust, servants of the AI god, Hellion, the principal villain of this adventure. Some of these gangs, such as the Lords of Rust, will almost certainly be enemies of the PCs. Others may just ignore the PCs. Some the PCs may even make into allies. But nothing is guaranteed to go in any particular direction. Redtooth's Riders, for example, are the group that the PCs will probably have the easiest time allying with, but even with them, they're just as likely to end up as enemies depending on how the PCs approach them. And what is particularly great is that the adventure's resolution is not dependent on the PCs becoming allies or enemies with any particular group.
The adventure does include a “scrap-worth” mechanic (named after Scrapwall slang), which measures the PCs' reputation in town. Various actions earn the PCs points towards their scrap-worth, and their total scrap-worth influences how NPCs and groups react to them. PCs who rush to face the Lords of Rust with only a little scrap-worth, for example, will actually face more opponents (not to mention also having less experience when they do so), whereas groups who wait to gather more scrap-worth before confronting the Lords of Rust will find that some of their potential opponents have actually fled out of fear of them. The scrap-worth mechanic is a good way for the players to see tangible results from their actions.
The idea of a town run by warring gangs may not be the most original concept ever, but there's more that makes Scrapwall really memorable. On top of a diverse selection of detailed, interesting NPCs, the town also has a rather unique visual appearance. Its name is actually quite literal, as Scrapwall is built out of a junkyard of old, discarded technology and other debris. It's like one big wall of densely packed scrap. “Buildings” are built into this wall, which completely surrounds the town and marks off regions and “streets” within. Combine this with the mishmash of science fiction and fantasy, such as orcs carrying chainsaws, and Scrapwall ends up leaving a vivid image in the mind, one that players won't soon forget.
Although there are numerous ways they can get there, the PCs will eventually go after the Lords of Rust and Hellion itself. Hellion, through the Lords of Rust, is in the process of excavating an old, buried excavation robot that it hopes to transfer its intelligence into and use to attack Silver Mount and Unity, the artificial intelligence that originally created Hellion (and has also gained divine power). One problem that can often occur in fantasy roleplaying adventures is that the PCs don't get much opportunity to interact with the villain. Generally, once they meet the villain, they fight and either the villain dies or the PCs do. This means that, no matter how interesting and detailed the villain might be, the PCs frequently never learn that. However, Lords of Rust makes use of its science-fiction elements to get around this problem. As the PCs move through the excavator to defeat the Lords of Rust and reach Hellion, Hellion frequently appears to them on monitors throughout, giving itself a demonic appearance (thus the PCs might be surprised when they end up facing a robot that looks very different). Hellion uses these appearances both to taunt the PCs and also to learn about them. The PCs similarly can talk back to Hellion. This allows the PCs to actually form a relationship of sorts with the adventure's villain before actually facing it. This is one of my favourite touches in the adventure just because this is something that, by the game's very nature, can so rarely happen.
Throughout the adventure, the PCs will hopefully also acquire clues that will lead them on to the next parts of the Iron Gods Adventure Path, particularly information about Hellion's sister, Casandalee. Given the open-ended nature of the adventure, however, GMs will have to do a little bit of work to make sure this information becomes available. Otherwise, the PCs could conceivably finish Lords of Rust with no idea what to do next.
As I mentioned, this volume does not have a support article about Scrapwall, something that one might normally expect. However, it really doesn't need a support article as the adventure details the town well enough. What the volume has instead is an article on the Technic League, the behind-the-scenes rulers of Numeria. The Technic League doesn't actually play a role in this adventure, although it will undoubtedly do so in later parts of the adventure path. However, the shadow of the Technic League does hang over Scrapwall as it does everywhere else in Numeria (one of the NPCs who is a potentially important ally could actually end up being an enemy if the PCs give her the impression they are from, or sympathetic to, the Technic League). This article, written by Jim Groves, gives a good insight into the workings of the League, including how people become members and rise through the ranks, and what the overall goals of the League are.
The second support article is another in the series of articles on the gods of Golarion, this one focusing on Brigh, goddess of invention. She is one of the more minor gods, but is particularly appropriate for Iron Gods as technology and clockwork fall under her purview. Also, one of the NPCs in the adventure is a cleric of Brigh. I've always found Brigh one of the more interesting gods and its nice to get greater detail about her here. I like that she favours the detached type of person who is more at home with her work and creations than with other people. This article also includes obediences for followers of Brigh who take the Deific Obedience feat (introduced in Inner Sea Gods).
Finally, this volume's Bestiary contains some new robots, an aberration, and a new type of undead called a rust-risen. Rust-risen are kind of like zombies merged with technology. Good fun.
Setting really is an important thing. While there is an important place for more generic adventure locales, often the adventures that players will talk about for years afterwards are the ones intrinsically tied into a strong setting with interesting and memorable NPCs. Lords of Rust has all these things while also managing to be a true sandbox in the middle of an adventure path. It is a very impressive adventure.