As any game designer knows, having a smooth difficulty curve is important in a game, as it makes the game flow well and allows the player to consistently be challenged. In linear games, implementing a proper difficulty curve is simple. Each subsequent part of the game just needs to be harder than the previous one. You can see this in action in any high quality linear game. For instance, in Portal, each test chamber gets harder and harder, culminating a boss fight. Because you know that the player has to follow a certain sequence, a game designer can up the difficulty as the player progresses through each area. However, in a non-linear game, where the player can go out of sequence or go anywhere they want, a smooth difficulty curve suddenly becomes tricky to implement. Let’s look at the solutions game designers have come up with to try and solve this problem.
One of the simplest ways to implement a non-linear difficulty curve is to not have one at all. That’s right, have all the content be at roughly the same difficulty for the entire game. This is the tactic used by The Legend of Zelda: Breath of the Wild. While there are pockets of stronger monsters, the majority of the game is about the same difficulty. Each divine beast is similar in difficulty, as is every shrine. The advantages of using this implementation is that it is simple to do and gives the player a feeling of getting stronger and mastering the game. However, in games with character progression, designers run the risk of the game becoming too easy, as the player can level or gear their character to the point where they can trivially complete content. This solution works in Breath of the Wild because there isn’t much character progression and good gear will eventually break, so the game becomes easy much more slowly. Becoming stronger is also tied to mastery of the game, so there is much more satisfaction to reaching the point of trivializing content. It also is a great fit for Breath of the Wild’s focus on exploration and discovery. Since the difficulty is constant, it enables the player to explore any part of the world at any time they want, giving the player true freedom.
Another simple way to have a difficulty curve is to scale all the content with the player. For instance, in The Elder Scrolls V: Skyrim, as the player levels up, so do all the enemies. This way, every fight and dungeon remains difficult regardless of the order visited. In contrast to having a constant difficulty, scaling content allows for the player to constantly be challenged and prevents content from being trivialized. However, because all the enemies get harder as the player levels up, the feeling of progression and gaining strength is mitigated. The player will always have trouble defeating enemies, which prevents them from feeling truly strong. So, while this method prevents the game from feeling too easy at later points in the game, it also doesn’t feel great to progress.
Instead of scaling content, a game can gate content instead. One common method typically used by metroidvanias is the idea of gating content behind player progression. This allows designers to put the harder areas behind locks requiring a certain item or ability.
While the player can explore and do some content out of order, there is an overarching linear progression of items or abilities that unlock additional, harder content. Compared to the previous two strategies, which allow for fully non-linear gameplay, games using gated progression are only semi-linear, allowing players to explore and complete some content in the order they want, but there is still some linear progression required.
Having tiers of difficulty allows the player to complete content out of order while still maintaining a general difficulty curve. In games with difficulty tiers, the player explores a group of similarly difficult content in any order before progressing to the next group of harder content. One example of this is The Legend of Zelda: A Link to the Past. In A Link to the Past, after the initial linear sequence, the map opens up allowing the player to complete the 3 dungeons in any order. Following another linear sequence, the player can choose between 7 dungeons before finally beating the last dungeon and the game. All of the dungeons in each of the tiers are similar in difficulty, which allows the game to have an overarching difficulty curve. However within a difficulty tier, the player often experiences the same problems as constant difficulty: the game starts getting too easy near the end of a tier. Similarly to gated progression, games that use difficulty tiers are also semi-linear, offering non-linear gameplay within each tier, but the progression between tiers remains linear.
In contrast to gated progression, where specific skills or items are required to continue, natural gating discourages players from entering higher level areas by simply making those areas hard. In Dragon’s Dogma, it is possible to wander into areas with monsters far above the player’s level. As a result, players who find that area too hard will avoid it until they are a higher level. However, the area is not forcibly blocked off, like in gated progression, as a skilled enough player can still complete the area. Natural gating allows players to still complete content out of order if they wish, but makes it difficult. Natural gating also has the side effect of allowing players to self-regulate their desired difficulty, as a player wanting more of a challenge can enter a high level area out of sequence. Compared to gated progression, games with natural gating remain non-linear as players can still do almost any content in the order they want.
The last way to have a difficulty curve in a non-linear game is hard to pull off. As you can tell by its name, mastery gating locks content behind player mastery and knowledge. The best example of this is a game called The Witness. The Witness is a non-linear puzzle game, allowing the player to do a variety of different puzzles in any order. However, some areas are locked by hard puzzles that require the player to have mastered or understood the mechanics of the puzzle. One excellent example of when a player experiences this is at the very beginning of the game. After a player leaves the tutorial area, they fairly quickly find this large puzzle with a bunch of black and white dots on it.
Without the knowledge of what the black and white dots mean and the mastery of the puzzles with black and white dots, the player cannot solve that puzzle. Instead, the player must explore the rest of the available puzzles and come back when they have mastered the black and white dot puzzles. Mastery gating is similar to natural gating, but instead of discouraging players from entering areas through high level enemies, it discourages players from entering by requiring mastery of a mechanic found elsewhere.
There are a variety of ways to add a difficulty curve to a non-linear game, each with its own benefits and drawbacks. Of course, some of the methods fit some genres better than others. For instance, I don’t think mastery gating works on much else other than a puzzle game or a platformer. Gated progression works best in metroidvanias and scaled content works best in RPGs. By understanding the different ways mastery curves work in non-linear games, you can pick the one that best fits your games. Or you might even discover a new way to implement non-linear difficulty curves. Who knows?