I didn’t realize how fun settlement building in Fallout 4 could be until I started working on an underground bunker filled with elaborate psychological experiments. Since its launch last year, one of the biggest additions to Fallout 4 has been the ability to construct small towns, little beacons of hope amongst the post-apocalyptic desolation. With the crafting system, you could build homes, shops, and entertainment, providing residents of the Commonwealth a safe place they could call home.

In the game’s latest add-on, called “Vault-Tec Workshop,” the entire experience is centered on this mechanic, but with one fascinating twist: you can finally build the twisted Fallout vault of your dreams.

Vaults are an integral part of Fallout lore. While on the surface the world has gone to hell following years of nuclear war, these underground bunkers remain isolated little pockets of humanity. And each is distinct from the next, thanks to Vault-Tec, the company that, in the fiction, built the vaults and used the apocalypse as an excuse to conduct vast, often bizarre psychological experiments on their dwellers. Some were home to machines that let residents live in virtual reality utopias, while others were meant to test the prolonged effects of cryogenic stasis. Mostly, though, vaults were isolated laboratories, perfect environments for scientists to mess with people and see what happens. In many of the Fallout games — including Fallout 4 — you’re put in the role of a vault dweller who escapes their bunker and ventures out into the big, wide post-apocalyptic wasteland.

At the outset of “Vault-Tec Workshop,” you’re investigating a distress signal sent out by Vault 88, located in the far south of the Commonwealth, Fallout 4’s version of Boston and its surrounding area. You have to do a bit of work before you can really get started — the vault is under attack by raiders, and swarming with not-so-friendly feral ghouls — but eventually you meet a Vault 88 scientist who, thanks to the wonders of nuclear radiation, has spent centuries dreaming of building the perfect vault. Naturally, she tasks you with helping her.

Like the rest of the previously released workshop add-ons, the story in “Vault-Tec Workshop” isn’t exactly deep or revelatory; don’t go in expecting anything near the scale of the fantastic “Far Harbor” expansion. Mostly, it’s a series of quests designed to get you familiar with the wealth of new construction tools at your disposal. Helping the scientist involves clearing out the massive space of Vault 88, luring residents of the wasteland into its chambers, and then creating a suitable place for the new inhabitants to live. Like all settlements in the game, you need to worry about the basic necessities — beds to sleep on, water to drink, defense systems just in case you’re attacked — but you also get access to seemingly mundane items that are perfect for entertaining or tormenting your new residents.

Mundane items that are perfect for entertainment or torment

The first is a simple exercise bike, one that can be used to help provide power to your vault. Later you’ll be able to build a slot machine to keep everyone entertained, a soda dispenser so they’re refreshed, and an eye exam machine for, you know, making sure they can see. But all of these devices can also be hooked up to a terminal where you can tweak their parameters. You can use the eye machine for its intended use, or, you can use it to show people a series of images intended to indoctrinate them into a life of indentured servitude. There’s a balance at play. Choosing the more humane options won’t elicit much of a reaction, which is bad for Vault-Tec’s experimental goals, but go too far and the residents will just leave. At one point the scientist got mad at me for trying to encourage bike riders to pedal faster through the use of fragrant oils, but when I switched to periodic electric shocks as a form of coercion, the population of Vault 88 decreased by one. It’s not all that hard to get more residents, however, and so it’s a lot of morbid fun just picking different experiments to see how people react.

These tools for torture are joined by a suite of new vault-themed additions to settlement-building. You can create rooms and hallways in that iconic vault shade of off-white, and new objects like some super-useful power generators make it look clean and efficient in a very retrofuturistic way. My favorite addition is a series of new motivational posters featuring the Vault Boy mascot and that cheeky Fallout sense of humor.

Perhaps the best part of the add-on, though, is the space: Vault 88 is absolutely huge. Once you clear the main area of debris — it’s filled with junk like broken-down trains and piles of rubble — the main area becomes this vast, open cavern that you can fill however you like. And once you get that settled, you’ll discover a labyrinth of subway tunnels that connect to it, each of which leads to even more usable space. It’s a bit of work to get everything in working order. Not only do you have to go through the often tedious process of scrapping every object for parts, but many of the tunnels are filled with dangerous predators like hordes of giant mole-rats and, in at least one instance, a massive, terrifying mirelurk queen.

It feels completely disconnected from the rest of the game

Ultimately, what makes this space, and this add-on at large, so great is that it feels completely disconnected from the rest of the game — just as a vault should be. Vault 88 is a place where you can get away from all of the troubles that plague the Commonwealth, and just kind of screw around for a while. You can make your settlement look just like any other vault, but with your own distinct layout, and then fill it with all kinds of weird experiments. It’s like a much deeper take on the mobile spinoff Fallout Shelter.

My vault is still in its very early stages — I’ve finished all of the “Vault-Tec Workshop” quests, but still have some big plans — but it’s really the first time that I’ve dedicated myself to the settlement building aspect of the game. And it’s because there aren’t any distractions, no new frontiers I can see just over the horizon. It’s just me, a vault, and the goal of making the most twisted experiments possible.

“Vault-Tec Workshop” is available now in all versions of Fallout 4 for $4.99.