It's impossible to talk about many mechanics present in the App Store today without talking about Puzzles and Dragons. As far as I can tell, this is the prototypical monster collection + fusion game.
P&D launched in February 2012 and according to the developer, it drew inspiration from card games. However, there are clear influences from Puzzle Quest (2007), which took Bejeweled's gem-matching mechanic and turned it into something that fueled spellcasting according to gem color. A set of equipment and quests supplemented the core match-three gameplay of the genre. There's also a call to the Shin Megami Tensei games (1994), which have long featured the concept of collecting and fusing monsters together to increase their power.
However, P&D couldn't be the powerhouse it is without their Free-To-Play mechanics. In order to progress, you need to gather additional monsters and these monster-gathering mechanics strongly resemble capsule machines (1). These impulse-buy vending machines might seem vaguely familiar to Americans who are used to American supermarket toy dispensers, but Japanese capsule machines are on another level entirely.
Capsule machines are similar to collectible card packs such as Magic: The Gathering or Pokémon. You put in money, turn the crank, and something pops out. It might be a rare, limited edition item. It might be an overwhelmingly common item. The random reward functions like a slot machine, and there are people (2) who will camp machines, pouring money in for that One Rare Item.
Certain mechanics may be novel to P&D, but it's difficult to tell given how widespread these mechanics are now. In particular, there is a friends mechanic where you select "helpers" (other players' avatars) to bring into battle. In some ways this is reminiscent of the Farmville model where you pull your friends into the game to get additional bonuses. P&D elevates this mechanic by creating a "friend coin" currency where using your friend's avatars gets you tokens used to summon slightly better monsters, encouraging you to request a friendship with every person you come across.
One way to measure Puzzle and Dragon's influence is by examining its UI and seeing how other games differ. For example, here's the "capsule machine" mechanic from P&D:
From Dragon Coins (May 2014, two years later):
From Monster Strike (Oct 2014):
There are countless more examples, but they all share similarities, aesthetic and functional. Limited-time promotional drops, a dragon-and-egg aesthetic, even the tabs at the bottom are all nearly identical across these three games. Each game lets you spend real money to purchase additional drops, but leaks out a small, steady amount of currency to keep you playing for free.
The similarities don't end there. The quest system rarely varies, focusing on timed and promotional content:
Fusing monsters together is identical:
All of these heavily repeated elements have some serious design flaws, though. The incessant focus on gambling-like mechanics puts this game pretty squarely in the “pay-to-win” category, where the quickest way to success is to dump money into promotional “egg” contests to get superior monsters. These superior monsters thoroughly outclass common monsters that drop via early missions.
Unfortunately, the game isn’t interested in teaching new players this lesson. The only way to learn that using fusion on early monsters will never get you to the same level of a rare monster is by understanding that this is a genre convention. The fusion mechanics tend to be opaque, never revealing the kind of stat increases you get via levelling or evolution. As a result, you are tricked into thinking you are progressing when in reality you receive pitiful stat bonuses.
Even the combat mechanics obfuscate how meaningful a monster’s statistics are. There’s no way to relate a +100 attack stat to the actual amount of damage you do to an enemy without carefully tracking numbers and puzzling out the algebra on your own. There are elemental bonuses where water is strong against ice, but “strong” is never defined – is it 1.5x attack? 1.1x? 2x? On top of that, when choosing a battle, you have no idea of the relative strength of the enemies versus your own team. Some missions will be labelled “hard” or “medium”, but that has no relationship to the strength of your team. It’s simply a static description that never gets updated. The only way to find out is to try and fail. Even comparing the relative levels of monsters doesn’t work because every “evolution” or step up on the power scale resets a monster’s level back to 1. This means a level 1 5-star enemy is much, much stronger than a level 5 1-star enemy.
Remember how Puzzle Quest was an influence on Puzzle and Dragons? Here's the latest version of Puzzle Quest:
There are plenty of similarities. The use of a limited-time promotion, token-based withdrawal system, with an option to pay real money to get more characters. There's a quest system that's partially time-gated. There's still the leveling aspect to collectible characters. Yet the interface and underlying mechanics are distinct enough to show that being inspired by Puzzles and Dragons doesn't mean mimicking it right down to the menu layouts!
In fact, Marvel Puzzle Quest fixes many of the problems with the traditional Puzzles and Dragons formula. While there is still an emphasis on stronger characters, even low-level characters can be useful as specific characters will get boosts for specific events. Furthermore, all characters are measured on a relative scale where a level-40 1-star character is roughly as strong as a level-40 4-star character. Leveling up your characters can only be done by collecting one of three character “covers”, where each of the three covers improves one specific ability. Events are rated “trivial” to “impossible” based on the levels of your team as compared to the enemy. For Player-Versus-Player match-ups, you can see your enemy’s team, their levels, and the strength of their abilities. You can even see the exact damage done by every character for every move they can make.
I don’t mean to come down too hard on Puzzles and Dragons. As I said, the influence it has on games even now is omnipresent and undeniable. However, many of the mechanics of Puzzles and Dragons feed into player fears about free-to-play games. They are grindy, opaque, and encourage paying real money in order to make any sort of progress. Yet even though the roots of Puzzles and Dragons do show that these fears are well-founded, games can still learn from the success of Puzzles and Dragons without necessarily falling into those same potholes. It’s important to understand these mechanics and what makes them work so that we can fix their flaws and thus make games that are not only more “fair”, but more interesting.
(1) It's hard to find an exact source for when these machines became popular in Japan. The best I can find is about 1965, but this leads to a Japanese Wikipedia page.
(2) Source: Personal conversations with people who have lived in Japan, collaborated on Twitter. Look, I'm doing the best I can, ok?