Beyond the Wall and Other Adventures was a slow burn for me. I bought it something like a year and a half ago, read it, thought there were some interesting ideas in there, but struggled to find the need in my life for another retroclone. It wasn't until the release of Further Afield that I decided to go back and check out the system more thoroughly. This involved downloading the numerous playbooks & supplements Flatland Games have made available for free on Drivethrurpg.
The system for Beyond the Wall is fairly simple. It's a typical retroclone with the same six basic stats they all have, a simple skill system using ability checks, and a magic system for levelless Vancian casting with rituals required for more powerful effects There are ten levels of progression, and three classes. These are presented fairly cleanly.
Where the system shines is in the procedures used to build the environment for play. There are several of these, but they break down into three categories: Playbooks, the shared sandbox, and scenario packs.
The shared sandbox denotes the area of the game world where the game takes place, and comes in two basic scales, which are integrated with one another. The first, contained in the core rules, is village generation. While using playbooks to create their characters, each player has the option to name and describe NPCs and locations in the hometown of the characters at certain points during generation. These may be related to the rolled result in the playbook, or not. The second scale involves generating the region the hometown is located in (rules on doing this are in Further Afield) and incorporating "threat packs". Each player goes around and gets to generate two major locations that their character knows something about (there are tables and rules that constrain this decision in useful ways). Players get to pick how they know about them (have they seen them personally, read about them in books, or heard local stories about them). This choice changes the roll the referee makes to determine how accurate the information is. Referees don't place major locations in this process, but they determine all the minor locations, and organise the map into regions.
Referees also get to choose to incorporate "threat packs", which is how they get to place major locations into the shared sandbox, as well as determining what the major threats facing the area are. Further Afield introduces the concept of a "threat pack", and comes with four examples (a dragon-boss, an imperial invasion, a magic kidnapper and a creeping blight). Threats have an "imminence" rating which slowly rises over time, and determines how active they are. They include one major location that the PCs may travel to deal with it, and typically suggest several minor locations. They have some information about overcoming the threat, bosses and monsters, an encounter table, relevant items or spells, etc. They also incorporate a table that at least one PC rolls on that determines how they are connected to the threat. This is a very clever concept on how to structure this kind of information, and it's something worth adopting into any sandbox game with large over-arching threats.
The end result of this process is a well-developed sandbox with lots of hooks and details that doesn't require a ton of referee investment at the front end.
Playbooks are the process whereby characters are generated. They're essentially a set of lifepath tables. Going through them, the players create NPCs and village locations as well as defining their relationships and shared history to one another (and generate their stats). Though theoretically there are three classes with multiclassing allowed, it's the playbooks that really develop the diversity here, with multiple playbooks to choose from per class, as well as multiclass playbooks that allow you to create rogue-mages and warrior-mages of different types. The playbook also includes most of the reference information that the player will need to play the game on the last two pages. There's currently somewhere around twenty playbooks by Flatland, mostly available for free. After play begins, if a character dies, a player can simply use the ordinary character generation rules to create a new character and add themselves back to the party.
The result of using the playbooks is to fill out the hometown of the characters and the relatonship between them with very little work needed by the referee.
Finally, scenario packs build on the work done before, and are the equivalent of adventures in Beyond the Wall. The ones released so far are written so that most of the relevant NPCs are actually generated from the list the PCs created during character generation (or from other NPCs added as the story goes on). Each scenario pack has a table of precipitating events that PCs roll on which give them a few clues, omens or warnings about what's coming. They then flesh out the various items, monsters, locations goals, etc. involved, often by using random tables that gives them a fair bit of replayability. They're a little more loosely written than typical dungeon adventures, but the information is presented so that the module is flexible, rather than vague.
The great strength of the system is how these different pieces work to feed into one another to diminish the demands of setting up and running a sandbox for referees and players. I've come to appreciate games that do this more and more (I am fond of Sine Nomine's games for similar reasons). I think I'm going to line up my next offline game to be Beyond the Wall because of this.