What Captive Minecraft is
Captive Minecraft is a custom survival map for Minecraft that traps you inside a world border one block wide. You spawn on a single square with the world closed in on every side, and the only way to grow your space is to earn achievements. Every new achievement pushes the border out by one block in each direction, so the world literally opens up as you play.
It was built by The Farlanders, and it has run through several versions over the years. The map turns the game's achievement list into a to-do list: mine some stone, cook some food, breed animals, visit the Nether, and each first-time achievement buys you a little more room to work with.
The premise
The border runs from bedrock to the top of the world, so you are never boxed in vertically, only horizontally. That leads to a lot of digging straight down and building straight up in the early game, when a few blocks of floor is all you have. Some achievements are easy and some send you on a real project, which keeps the pace interesting. There is a catch in the Nether too, where the border is much tighter than in the overworld, so a careless step can end a run fast.
Why it works for multiplayer
Captive Minecraft is more fun with a group. Achievements only count the first time anyone earns them, so a team naturally splits the work: one person farms, one person mines, one person heads for the harder milestones. You are all packed into the same small space, which means you are constantly on top of each other, trading items, and arguing about what to unlock next. The map even tracks who has earned the most, so there is a friendly scoreboard baked in.
How we played it
On BasketMC we played Captive Minecraft 4 as a series called Release the B.S., with the Team B.S. crew crammed into one tiny border and working through the achievement list together. If you want the full run, that series has its own page with the episode list.