Designing a Sustainable Game Economy: Card Scarcity and Mining via Play

Luke Zhang
Sep 19, 2018 · 9 min read

When we started building Zombie Battleground, two of the fundamental principles we set were that the game should be free to play and not pay to win.

But at the same time, one the major advantages of blockchain games is the ability to freely buy and sell truly rare items with other players. So we also wanted to create a thriving in-game economy where players are excited to trade on the Marketplace.

This presented a problem:

How is it possible to create truly rare assets that people want to collect without giving an unfair advantage to the players who spent the most money?

We eventually found the solution to this problem, strangely enough, within Bitcoin’s mining algorithm.

…but I’m getting ahead of myself.

Let’s start with some background…

Free to Play — For Reals

From the beginning, we decided on principle that we wanted Zombie Battleground to be truly free to play.

Forcing people to spend hundreds or thousands of dollars on card packs in order to be competitive (like many games do) would be cutting the game off from a large percentage of people who are unable or unwilling to spend that much money on a game. And that didn’t fly with us.

Unfortunately for most gamers, these days even most “free to play” games have typically also come with hidden cash grabs, such as loot boxes and disproportionate gameplay advantages given to paid players.

So first, I want to clarify what we mean when we say free to play:

  1. A gamer who plays Zombie Battleground regularly for a couple of months should be able to earn a competitive deck entirely from rewards — without spending a single cent.
  2. No card should offer a competitive advantage that’s available only to players who pay money.
  3. Anything that affects gameplay should be earnable simply by playing the game.

The reason these qualities are important is because it’s the only way to build a long-term sustainable game economy.

To understand why, imagine we made a powerful card that was only available to those willing to pay $500 for it.

In order for this card to have value on the marketplace, it would need to be scarce — if it were unlimited in supply, eventually supply would outstrip demand, and the card’s value would trend toward zero.

But if we create scarcity by printing these in limited quantity — say only 1,000 of these cards will ever exist — then any player who does not own one of these cards would have a serious handicap in gameplay.

This would kill the fun factor of the game, and disincentivize new users from wanting to play. Who wants to play a game where they get stomped by veteran players with super rare powerful cards that they have no way to acquire without shelling out a ton of money… or that they can’t acquire at all because no one is selling them?

Not a very good user experience.

Earning Cards by Winning Ladder Matches

The main way to earn cards for free in Zombie Battleground is by playing ladder matches (which will be enabled during the public beta release later this year).

For every match you win, you will earn card packs as rewards.

We estimate that, on average, each game will take approximately 10 minutes. So a gamer should be able to play 6 matches every hour — that’s up to 6 chances per hour to get cards without spending a single cent.

This includes cards from any rank — from the common Minions even up to the rarest Generals.

The exact distribution is still undergoing modeling to ensure a viable economy, but here are the current odds we have in the alpha version of Zombie Battleground:

Odds of receiving card types in each pack earned in Zombie Battleground alpha version (subject to change)

So in essence, the economy grows with the number of players and the number of matches played — with each additional player, additional cards are minted and added to the economy.

Truly Rare “Limited Edition” Cards

So how do we create lasting value in such an inflationary economy?

This has already been solved by games like Magic: The Gathering — rare collectible cosmetic variants.

In a presentation by Magic: The Gathering’s head designer Mark Rosewater, he talks about how one of the most valued cards in Magic is a rare cosmetic variant of an Island — one of the most basic land cards in the game.

This card offers absolutely zero in-game benefit — it’s exactly same as every other Island card.

Yet due to its rarity and cosmetic uniqueness, collectors are willing to pay significant amounts of money to own a copy of this card. Players like to pimp out their decks with cool-looking cards.

Magic also has foil cards — which are stylized versions of normal cards that also offer zero gameplay benefit, but simply look cooler and are very rare (only one foil card for every six packs).

League of Legends is similar — the game’s entire $1 billion business model relies on players buying optional cosmetic skins of characters and items that give them zero in-game benefit… they simply look cool.

In Zombie Battleground, we have “Limited Edition” cards, which have unique looks (such as animated images), non-gameplay benefits (such as increasing drop rates of more Limited Edition cards in the future), and most importantly, true scarcity — these cards only exist in a fixed quantity, and once they’re all acquired by players, they’ll never be created again.

This is the placeholder image you’ll see if you get a Limited Edition card when you open your card packs. The final versions of these cards will be animated, but since the animations aren’t finished, for now they simply display a shining zombie embryo.

Naturally, since these cards only exist in a fixed quantity, their value will increase over time as more and more players join the game and demand increases with supply staying the same.

This system allows us to have truly rare tradable items that promote a healthy trading economy, without ruining the game’s playability by giving these paying players unfair advantages.

Mining via Play: Using Bitcoin’s Algorithm to Mint Truly Scarce Game Items

How do we control the supply of Limited Edition cards to distribute them over time? And how do we deal with users trying to create many accounts to spam the network and mine all the Limited Edition cards before anyone else?

It turns out that these problems have been already solved by Bitcoin.

In Bitcoin, people who process transactions and help secure the network are called “miners”. For their services, the miners are rewarded bitcoins for every block mined.

The number of coins mined per each block is fixed, which means as more and more miners join the network, the mining difficulty increases, and the chance of each individual miner earning the rewards is reduced.

This has the effect of causing bitcoin’s value to go up over time as more and more miners compete for it.

Zombie Battleground Works the Same Way

Each Limited Edition card is set in a smart contract to have a fixed quantity that will ever be minted, and a fixed number of this card that will be “mined” per each time period.

When you start winning ladder matches, in addition to the free “Standard Edition” cards you receive, you’ll also be mining for Limited Edition cards.

Since the difficulty of finding these cards increases as the number of Zombie Battleground players increases, the value of these cards will also go up as more and more players mine for them.

In this way, players who spend a lot of time playing the game are rewarded — especially the skilled players who help make the game more interesting and more competitive for everyone.

Rewarding High-Ranking Players with Better Rewards

Not all accounts mine Limited Edition cards at the same speed — as a player’s ladder ranking increases, so do their odds of mining Limited Edition cards.

This has a number of advantages:

  1. New players should first focus on earning enough “standard” cards to build a competitive deck and learning the ins and outs of the game before they can start earning collectors items.
  2. The system rewards competitive players who spend a lot of time making the game interesting and more competitive — even giving them a way to earn money by selling these rare and valuable cards on the Marketplace.
  3. It serves as additional resistance to bot attacks, because a large number of low-skilled accounts will earn less rewards than a single highly skilled player.

Reward Halving

In Bitcoin, the mining reward halves periodically, and will eventually become zero — at which point a grand total of only 21 million bitcoins will ever exist.

Image result for reward halving
Bitcoin’s Reward Halving makes it so the rewards are highest in the beginning and keep halving over time, until eventually no new coins are minted.

This same mechanism applies to Limited Edition cards in Zombie Battleground. This means each Limited Edition will only exist in a fixed quantity, and will slowly enter circulation over time.

Every season, a new set of cards will be introduced, and on launch day, the mining difficulty will be at its lowest — which means the chances of finding the latest set of Limited Edition cards will be the highest.

Over time, as the reward halves, that Limited Edition will be harder and harder to find, until it stops being minted altogether. New seasons will overlap with the previous season, ensuring a constant flow of new Limited Edition cards for active players.

But because rewards are highest at the start of each season, it’s always a great idea to increase your playtime as soon as a new card set is released — that’s when you’ll have the highest chance of getting these new cards.

Consider that tip to be your reward for reading this post all the way down to here 😉

Fuse Cards Together to Upgrade Them

Does this mean that “standard” cards will not have much value?

Quite the opposite — we’ve designed the game with a Fusion mechanism that allows you to merge multiple Standard Edition cards to form upgraded (and even rarer) versions.

The Card Fusion mechanism allows you to upgrade multiple Standard Edition cards by fusing them together.

Not only does this add a fun element by giving players something useful to do with all the Standard Edition cards they’ve earned by playing a lot of matches, but it also serves as a downward force on an inflationary economy, taking “normal” Standard Edition cards out of circulation and replacing them with rarer cards.

These upgraded Standard Edition cards can also be fused with Limited Edition cards, upgrading the non-gameplay abilities of those cards (e.g. upgrading a Limited Edition may increase your drop rate for Limited Editions from an extra 1% to an extra 1.5%).

There will be multiple tiers of fused upgrades available for each card, which we will be detailing in a later update.

What Does this Mean for Early Backers?

During the closed beta period, the ladder won’t be open, since we are in the process of balancing the game. This means players won’t be able to earn pack rewards yet.

Mining via Play for Limited Editions won’t be in place during this period. The card packs that players purchase will simply have a fixed chance of 0.2% for having those cards upgraded to Limited Editions, giving them a head start on acquiring these rare cards before mining is enabled.

Cards opened during the alpha will also have a 15% chance of being upgraded to Backer Edition cards — another rare cosmetic variant which will never be printed again after the closed alpha ends, providing additional collectors items to those who backed and supported us during these early stages.

Lastly, there are the “Kickstarter Exclusive” cards that were sold to our earliest backers. These cards are even more rare and exclusive than regular Limited Edition cards, since they will only exist in the quantity purchased when the Kickstarter ended, and will never be created again.

What if I Only Care about Trading?

Instead of thinking of trading as a supplementary part of Zombie Battleground — we’ve decided to treat trading with as much importance as battling zombies.

In fact, many of you might heavily focus on becoming a trader and spend your time only on the Trading Post.

In a future article, I’ll be talking about:

  • How traders can level up and earn additional rewards
  • How traders can form a guild with players (and share card rewards!)
  • …and a lot more cool stuff

But — you’ll have to be patient until I reveal those later 😉

Happy battling!

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Luke Zhang

Written by

Co-Founder at Loom Network

Loom Network

Loom Network provides developers the scalability and usability they need to build high-performance user-facing dapps today. With seamless integrations to Bitcoin, Ethereum, Binance Chain, and all major blockchains, deploying once to Loom lets you future-proof your dapp