In DF Felltower, the players have access to their own meta-knowledge of the game, usually from memories of play, but also from my mostly-reliable but not exactly detailed session summaries. Anything that happens in Felltower that makes the summaries, plus some details that do not, are generally known. Every person on the street might not know them, but they could - nothing is hidden. PCs aren't allowed to "hide" knowledge or conceal discoveries or hold back information to bargain with. It's all out there . . . someone blabs, always. You don't need to justify knowing, as much as you need to justify not knowing.
This doesn't mean it's actionable knowledge . . . knowing you found a gate doesn't mean everyone knows how to get there and just goes. But it might - if any group of PCs, including one assembled entirely from new PCs with no connection to existing members can use the map, then such knowledge could also be out there and known by others. In any case, this is how rumors get generated, and why details about things discovered tend to come up. It's assumed as a basic fact of the campaign that people talk, and then NPCs in town talk back based on what they heard.
Characters may come and go, but the blog remains, and even players who started long after play began can go back and read them and use what they know to inform their decisions. The effects of their decisions persist. This includes memories.
Players can use their memories to inform their decisions. PCs may see something for the first time - they don't remember the thing, or the room, or the event - but they can know about it.
This is a two-way street. The players can use all of the summaries, and all of their own memories, even those of PCs long dead, to make larger decisions about the game. In a way, the characters have a group identity. But therefore NPCs tend to regard the PCs as a group, too. They learn about how murderhobos (wether evil sorts like the Barca family, or religious zealots like the current group) act and behave, and react accordingly. You may not have electrocuted Larry the Crossbowman, but he's learned to stay away from most types who'd go into Felltower delving after loot.
Basically, the players can remember stuff and try to apply that to their PCs. The PCs don't have supernatural memories of other lives, but can have knowledge of previous events. And NPCs tend to learn lessons about PCs as a whole, even if some new PC does get a chance to change how things work. How your PC treats an NPC is how that NPC will expect all PCs to treat him, it, or her in the future. They may be swayable by a new PC with better social skills, or betrayabe because the last group was trustworthy and this one is not, but you don't get a clean reset like you've walked far enough away from an NPC in Skyrim and come back and they've forgotten the arrows you shot at them.
This is probably the weirdest law of Felltower. But it's strange to do this one way, but not the other. In other words, if the players act as a stable group with rotating members but a single, unified map and summaries and access to the blog, but the NPCs and monsters only react to individual PCs based on that individual PC's actions and don't know or learn based on that . . . it feels lopsided. And very, very game-y (and video game-y) in the worst possible way. A one-way approach like that just doesn't fly. It makes it very easy on the PCs - anything they know, they know - and very hard on the GM - anything the NPCs know, they don't know unless it's about a specific PC that they directly interacted with.
That doesn't fit with my sense of fairness or my sense of laziness.
So this law applies.
Descriptive, not Prescriptive
It's worth nothing that these "laws" are descriptive, not prescriptive. Any player reading these should take them as basic descriptions of play. They should not take them as rules you can leverage for your benefit. They don't insist that things are thus or so. They merely describe, in roughly accurate terms, how things have been and generally are. Pushing them for a benefit isn't a good idea - it's not that kind of game, and these posts are merely trying to describe some of the assumptions those who've played the whole time have learned unconsciously or consciously along the way. Much like grammar, it'll tell you when something is wildly wrong, but it's tough to say some new development in the common language of Felltower isn't proper just because it doesn't quite line up with these. Keep that in mind.