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Behind Back 4 Blood's ingenious card system

"The heart of the cards" isn't just for teenagers with anime hair anymore.

Back 4 Blood, Turtle Rock's latest addition to the co-op zombie shooter genre, gives a fresh take on everything players have enjoyed about Valve's Left 4 Dead series for the last decade. It's got weird zombies (okay they're called "The Ridden"), gonzo objectives, great gunplay, and an AI "director" that is deliberately out to ruin your day (or give you some health packs--as a treat).

But there's one huge mechanic that wasn't in the Left 4 Dead games that has helped make Back 4 Blood an incredible successor to the original game--the card system. After completing runs and unlocking rewards, players can acquire new cards to put into a "deck" that can be drawn as a run through Back 4 Blood progresses. These cards can provide beneficial stats, rewrite abilities, and more.

Additionally, the AI director can also throw cards into a deck during the player's run, acting as another lever to mix-up the Back 4 Blood experienceWhen the core hook of a game is being able to run the same levels over and over again--it's a great tool for keeping that experience fresh.

We wanted to know more about how Turtle Rock cooked up this neat system (and why the heck did they go with cards?), so we reached out to lead game designer Brandon Yanez to discuss the system's origins, what works well about it, and what the team wants to do with it in the future.

Deckbuilding and roguelikes

First, a quick primer on how the card system in Back 4 Blood works.

When players start the game, they are given a small pre-made deck of cards that contain different active and passive buffs for themselves and their teammates. At the beginning of each act, players "draw" cards and select ones they one to activate for the run. As players earn supply points, they can purchase more cards by spending those points on different "supply lines." Each supply line contains cards that are meant to synergize.

Players are only given 7 cards at the game's start, but can build decks of 15 (more on that later). At the beginning of each campaign, the AI Director will add some number of "Corruption Cards" to the deck, which can appear and impact player runs.

Yanez explained to Game Developer that the card system emerged out of the design team's fascination (and love) for the roguelike genre. In the last few years, games like Slay the Spire and the recently released Inscryption have mixed deckbuilding mechanics with different kinds of combat to great effect, giving players (and designers) a large number of levers to pull when customizing different experiences.

Turtle Rock co-founder Chris Ashton was one of those team members, and he and Yanez were among the first to realize some variation of a deckbuilding system would be a good fit--though the team didn't necessarily stick to calling them "cards" right away. "We were actually a little hesitant to use cards, too, because we didn't want any sort of misunderstandings on what it was, or how we were going to sell [them]," he explained. "It took us a little while to understand that cards were a good fit for us."

A screenshot of Back 4 Blood's Deck Managment screen, showing cards like "Ammo Stash" "Durable", and "EMT Bag"

That's because when they were explaining the concept to other team members, cards and deckbuilding were just the right metaphors to use. "Why try to force it into something else when it just makes sense?"

Yanez said that the idea got additional traction once they realized the AI Director could be wrapped into the system. It's one thing for players to know a level and build a deck anticipating its challenges, it's another for them to have to react to the Corruption Cards as well. 

From there, the team needed to learn what actually a "good" card felt like. What surprised them was that strong cards that gave them access to more items, with a clearly trackable increase in power or utility, didn't go over well. But game-twisting effects that changed the rules went over like gangbusters.

"Now your [melee attack] gives you a knife, now you can shoot while sprinting--these talents or abilities tend to actually register," said Yanez. "Stat-based stuff wasn't so good. It takes a very savvy player to know that a 5 percent increase in move speed is pretty big."

Though stat increases are part of Back 4 Blood's final system, it's worth noting that the "Combat Knife" card is one of the first ones players receive.

Supply chain issues

As Yanez noted above, Turtle Rock had little desire to introduce a booster pack or IAP system that would randomize card distribution. Figuring out how to get cards into players' hands--and forcing them to experiment with them--turned out to be something of a challenge without the tools that have worked so well for Magic: The Gathering or The Pokémon Trading Card Game.

"We started with [card distribution] being fully random at first," Yanez explained. "And then that didn't really sit that well, because players would have different experiences with with random progression."

This led to the creation of the Supply Line system--a kind of technology tree-adjacent process where players can spend resources to gather cards down certain build paths. Players have access to three of them at the jump, splitting out the cards into roughly three different playstyles that help them navigate the campaign's challenges.

There was still a need to keep some high-value cards buried deep in progression. "If you get every card that you wanted up front, then it takes you a while to want to search out cards that you wouldn't normally go for," Yanez said.

And if players aren't hunting for cards, they're not as likely to experiment with the more "out there" cards Back 4 Blood has to offer. 

There was another issue with getting players to experiment as well. Per Yanez, early playtesters among the development team didn't feel very invested in experimenting with any early builds they had access to. One common piece of feedback the team got was "I just want to shoot zombies, man."

A zombie covered in sharp spines leads a hoard of undead.

But Yanez said he had a hunch that futzing with cards would be "sticky", and didn't take this as a sign the system needed to be dumped.

The solution was kind of an inverse supply/demand approach. Back 4 Blood's starter deck doesn't have its full 15 cards--it has plenty of space for players to immediately start adding new cards they acquire after their first run.

"So you'll get a new card, you'll put it in there and you'll be incentivized to try it because you don't even have the full 15," Yanez said. 

This helped dispel the notion that players need a "full deck" to start playing Back 4 Blood as the game's core mechanics obviously revolve around moving and shooting, not power parity between a player's deck and the AI. Though there's obviously some balancing needed to ensure players aren't positively wrecked on their first go, this solution has some neat design subtlety. 

Games that use deckbuilding--but don't use cards as the core mechanic--can shrug off some of the necessities of other card-based games.

Monstrous ideas

With the card system revving along at full speed, its designers could be consistently surprised and inspired by the rest of Back 4 Blood's creature and level design. Yanez said that many cards in the game are different kinds of "happy accidents," emerging from playtest sessions. 

The addition of creatures like Crushers and Hockers (who can each lock down players and restrict movement) led to the creation of the stun gun--a limited-use "get out of jail free" mechanic players could invest in. But with the card system in hand, the developers began thinking about other "breakout" systems that could exist as cards.

That led to the creation of the breakout character Evangelo, Yanez explained. Back 4 Blood's character selection interacts with the decks, and players gain access to new characters by unlocking their cards.

Yanez himself enjoys playing support class roles, especially when helping the team test Nightmare difficulty. He uses cards like "Charitable Soul" to heal his teammates and himself in one go. His colleagues on the Nightmare balancing team use the cards to carve out different unofficial roles. 

One uses an inventory-expanding card that benefits the entire team--who then pass their grenades back to him so he can use them to blow holes out of tight spots. Another takes cards that let her debuff Elite Ridden, creating a homebrewed class designed to seek out the tougher enemies and mark them so that the team can shred them to bits.

Now that Back 4 Blood is out in the wild, Yanez says he and the team are learning what players are responding too en masse, and it's helping them create new cards for the future. 

However weird it may be to describe a zombie-shooting game as sharing the same mechanics as Magic: The Gathering, it's gone over great for Turtle Rock studios--and it'll be intriguing to see where the system and its designers go from here.

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