Case Study 25.1 — Hollow Knight's Charm System: Building Identity Through Constraint

Game: Hollow Knight (2017) Studio: Team Cherry Designers: Ari Gibson, William Pellen Platforms: PC (original), Switch, PS4, Xbox One, and subsequent platforms Why it matters: Hollow Knight shipped with one of the most elegantly constrained progression systems of the 2010s — a charm system that looks modest on paper (about forty collectible charms, each granting a small effect) and produces, in practice, the single strongest mechanism for build identity I have ever seen in a Metroidvania. The design lesson is not about size or complexity; it is about the specific constraint Team Cherry imposed — the notch limit — and how that single decision transformed a straightforward buff-catalog into a system that forces meaningful tradeoffs on every engaged player. This case study is about how constraint enables creativity, and about how horizontal progression can replace a traditional skill tree without losing identity.

What the System Is

The Knight starts the game with three notches in a charm bar. As they progress through Hallownest, they acquire more notches — eventually, through persistent exploration and puzzle-solving, the Knight can accumulate eleven notches. That is the ceiling.

Charms are items the Knight equips into the notch bar. Each charm provides a small effect: a damage boost on a specific attack, extra health (masks), faster healing, a passive that triggers on low health, a counter-attack effect, a ranged attack. Some charms are utility (a compass that shows the Knight on the map), some are offensive (a charm that boosts nail strength at close range), some are defensive (Stalwart Shell, which extends post-hit invincibility). A few are overtly weird (Gathering Swarm sends a little cloud to collect dropped currency for you; Fragile Strength raises nail damage but breaks on death).

Each charm occupies a number of notches proportional to its strength. Small, situational charms take one notch. Strong charms — the Shaman Stone that boosts spells, the Quick Slash that raises attack speed — take three. A few of the most powerful charms, like Grubsong or Steady Body's tier upgrades, take more.

The math is simple: your equipped charms must fit into your notch bar. A player with three notches picks at most three one-notch charms, or one three-notch charm, or any combination that totals three. A player with eleven notches has more room but still cannot equip every charm — the total cost of all charms in the game far exceeds eleven.

This is the entire system. There is no skill tree. There is no XP curve-driven stat progression. There are no talent points. The Knight's combat capabilities grow slowly through a handful of nail upgrades and spell acquisitions; beyond that, the variation between one player's Knight and another's lives entirely in the charm selection.

What the System Forces the Player to Do

Here is where the design gets interesting. Because the notch limit is tight, the player cannot equip everything. They must choose. And because different encounters reward different charm choices, the player cannot choose once and be done — they must re-choose, repeatedly, as the game presents new challenges.

A few specific cases illustrate the texture.

Approaching a boss. The player is about to fight the Mantis Lords. The fight is fast, highly mobile, and rewards precise nail hits. The player looks at their charm bar. They have Quick Slash (three notches, attack speed up), Mark of Pride (three notches, nail reach up), Grubsong (two notches, SOUL regeneration on hit), Shaman Stone (three notches, spell damage up), Stalwart Shell (two notches, longer invincibility after being hit), and others. They have nine notches.

This is already more than fits. They choose. Quick Slash and Mark of Pride fit together in six notches and dramatically improve the core nail attack the fight rewards. Add Stalwart Shell for safety — eight notches, one left. A single-notch charm in the remaining slot? Maybe Gathering Swarm to pick up any dropped currency during the fight. They equip, run to the boss gate, and enter.

They die. The fight is fast and Stalwart Shell is not helping as much as they hoped. They retreat, reconsider. Drop Stalwart Shell, pick up Grubsong (two notches) for better soul generation — now they can heal and use spells more aggressively. Re-enter.

This micro-optimization loop is the progression system. Every major encounter is an opportunity to re-plan the charm loadout. The planning itself becomes engaged play. The player is not grinding XP; they are doing tactical inventory management that matters.

Exploring vs. combat. The game has long exploration stretches where combat is intermittent and the challenge is navigation and puzzle-solving. Here the player might swap to Gathering Swarm, Wayward Compass (shows position on map, two notches), Hiveblood (regenerates the last mask of health over time, three notches) — an exploration loadout. When they hit a combat-heavy area, they re-equip the combat loadout.

The edge case: overcharming. The system permits one transgression. A player can equip a charm that exceeds their available notches as long as the overflow does not exceed the cost of any single equipped charm. The catch: overcharmed, the Knight takes double damage from all sources. Some players see this as a risk-reward option (an extra charm at high cost); others see it as a buff for confident players who will not get hit anyway. The overcharm mechanic is almost a meta-design statement: the notch limit is real, and breaking it costs you.

Why the Constraint Is the Genius

A game with forty charms and no notch limit would still produce variation — players would equip their favorites and ignore the rest. But the identity would be weaker. Without a limit, a player can equip every charm they acquire, and the "build" is whatever charms they happen to have found. There is no choice, only accumulation.

The notch limit forces choice. And choice is what produces build identity.

Here is the point I want to make: most game systems with complex character customization rely on some kind of scarcity to produce identity. XP points are scarce — you cannot allocate everywhere. Gear slots are scarce — you can only wear one helmet. Talent points are scarce — you can only take so many per level. These scarcities are what make the choice of "what to invest in" feel like a choice. Remove the scarcity, and the system becomes a checklist.

Team Cherry's innovation was making the scarcity the central mechanic. Not as a side-effect of earning rates, not as a function of a cap somewhere, but as the visible, always-present constraint that the player interacts with every time they open the menu. You cannot forget you are constrained. The bar is right there, eleven notches, and your charms are visible in their slots, and every new acquisition is a question: does this replace something, or does it wait in the inventory for a future loadout?

This is horizontal progression without stats inflating. Each charm is different, not stronger. Unlocking a new charm is a widening of the player's expressive range, not an escalation of their damage numbers. The Knight at hour thirty is different from the Knight at hour three in what they can become when the player re-equips, not in their underlying power. Except for a few nail upgrades, the Knight's peak damage at endgame is not dramatically higher than mid-game — the improvement is lateral.

Combine this with the excellent skill-growth curve (the player learns to read bosses, to pogo, to conserve soul, to manage spacing) and you have a game where all the meaningful progression is horizontal and player-skill based. The numerical character growth is minimal. The build variety and the skill growth are enormous. Hollow Knight is, in the ratio we discussed in the chapter, something like 85/15 player-skill to character-power. It feels deeply progressive anyway. The charm system is the mechanism that makes horizontal progression feel equivalent to vertical progression — because every major encounter re-engages the build decision.

💡 Intuition: The notch limit is not a UI constraint. It is a design statement: the Knight is small and limited, the player's cleverness is what makes them capable. The system rewards thought over investment. The genius is that the constraint feels in-fiction — the Knight is a little bug with a small body, of course they cannot carry every trinket. Team Cherry made the game's core progression apparatus feel like an emanation of the character's physicality. Mechanics and fiction are one.

How Team Cherry Tuned It

The charm system did not ship in its final form. Early playtest feedback drove several specific adjustments.

Notch counts per charm. Several charms were re-costed after playtest showed they were either dominant or neglected. The Mark of Pride was originally cheaper; play-testers reported that every build included it, which meant the cost was too low. After a cost raise, the charm became a meaningful "I am committing to nail range" decision rather than a default inclusion.

The overcharm mechanic. Without an overflow mechanic, players who found a powerful high-cost charm (Shaman Stone, Quick Slash) late in the game had nowhere to put it without ripping apart their existing loadout. Overcharming let a player include one powerful charm at the cost of vulnerability. This was a tuning-for-edge-cases decision that made the late game more flexible.

Charm distribution. The order in which charms are acquired is deliberately staggered. Early charms are low-cost and versatile (Wayward Compass, Gathering Swarm, Stalwart Shell) — they teach the player the system without overwhelming them. Mid-game charms start to offer real trade-offs (Mark of Pride vs. Longnail, both three notches for similar effects but different strengths). Late-game charms force hard decisions (Fragile-series charms, which break on death and require repair — a risk layered onto the charm decision).

Synergies between charms. Some charms are designed to combo: Grubsong plus Quick Focus (faster healing) plus Stalwart Shell produces a survivor build. Spell Twister plus Shaman Stone plus Soul Catcher produces a caster build. The designers recognized that emergent synergies would be part of the community's theorycrafting, and they tuned the synergies deliberately — buffing combinations that felt fun, nerfing combinations that trivialized fights.

No respec penalty. Charms can be un-equipped freely at any bench (the game's save-points-and-rest-spots). There is no cost to experimentation. This is critical: a locked charm loadout would make players afraid to try new configurations. Team Cherry understood that the system's value came from re-equipping frequently, so they removed friction from the act.

Pacing of notch acquisition. The player does not find notches on a predictable schedule. Some are found exploring; some are rewards for specific sub-quests; some are hidden behind significant challenges. The acquisition of a new notch is a progression event in itself — it is always a moment, never a routine. This keeps the notch bar feeling like a cherished resource, not a background number.

What This Replaces

One way to see the charm system is as a replacement for the traditional skill tree.

A skill tree, as the chapter noted, is a graph of nodes with prerequisites, often representing a commitment to a specialization. You spend points to walk a path. By late game, you have a character whose build is defined by the nodes you took, and switching to a different build requires a full respec (or sometimes a whole new character).

The charm system does similar work through different mechanics:

• Build identity without a tree. Every player's current charm loadout says something about them — aggressive, defensive, exploratory, technical. Two players at the same point in the game can have radically different active charms.
• Specialization without locking. A player can be a caster build today and a damage-dealer tomorrow. The specialization is in the current loadout, not in irreversible allocations.
• Growth over time through breadth, not depth. New charms expand the player's option space rather than raising numbers.
• Meaningful decisions because of the notch cap. You cannot have it all.

The cost of replacing a skill tree with a charm system is that the progression does not "stack" in the way tree nodes do. A skill tree produces a long vertical line of accumulated commitment; the charm system always fits in eleven notches, so even a veteran player is playing within the same constraints as a new player. For some audiences this is weakness (less sense of hard-won accumulation); for others it is strength (no one feels "too far behind" — the best loadout is available at any skill level).

For Hollow Knight's intended audience — players who want a tight, focused Metroidvania with a tough-but-fair difficulty curve and deep replayability — the charm system is arguably superior to a tree would have been. A tree would have added numerical bloat, eroded the tight mechanical focus, and turned the game into an RPG. The charm system preserves the game as an action-adventure with tactical inventory.

🎮 Case Study Note: Other games have borrowed elements of the charm system since. Dead Cells uses a similar mutation system, though with roguelike rerolling. Rogue Legacy 2 uses equippable traits. Gunfire Reborn uses a scroll system. None has the same cleanness — Hollow Knight's system works because it is not layered on top of other progression systems; it is the progression system, with minimal noise. The integration is total.

Lessons for Your Own Work

You probably are not designing Hollow Knight. You are designing a smaller prototype, possibly in a different genre. What can you take from this case study?

Constraints are where identity lives. If your progression system lets the player have everything eventually, it will produce homogeneous characters. Introduce a scarcity — notch-like, slot-like, mutually-exclusive-choice-like — and identity will follow. The scarcity does not need to be punitive. It just needs to be visible and present in the player's interaction with the system.

Horizontal progression can carry a game. You do not need numerical character growth to produce the feeling of progression. If your design is tight enough (as Hollow Knight's is), lateral option-expansion plus player-skill growth is plenty. This frees you from tuning XP curves, scaling enemies to levels, and managing power creep across content releases. It is not easier to do well — but it is a different set of problems, often more tractable for small teams.

Integration beats accumulation. A single system that expresses the game's progression (charms + nail upgrades + spells) is stronger than multiple parallel systems that each do a piece. When the charm system is the progression system, every charm decision matters. When progression is spread across level, gear, skills, charms, perks, talents, essences, and sigils — which is where many modern RPGs end up — no single decision feels load-bearing, and the player experience fragments.

Re-engagement beats investment. A skill tree is a one-time allocation. A charm system is a constant re-engagement. Design so that the player revisits the progression decision regularly, not just in character-creation. The more often the player touches the system, the more alive it feels. Hollow Knight's benches are where the entire game re-plans itself, every time.

Let fiction and mechanic converge. The notch bar is an in-fiction constraint on the Knight's body. The charms are in-fiction objects with histories and lore. When the progression system feels like a property of the world, not a UI layer on top of it, it integrates. Every major mechanical progression in your game should be asked: what does it mean in the fiction? If the answer is "nothing," the mechanic is weaker than it could be.

Closing Thought

Team Cherry built Hollow Knight over several years with a tiny team (three people, for much of it). They had no budget for an RPG-scale progression system. They invented one that works because it is small, focused, and constrained — because the constraint is the design. Eleven notches. About forty charms. Total charm cost across all forty far exceeds eleven. Everything else follows.

This is what practitioners mean when they say "constraint enables creativity." The notch limit did not handicap the design. It made it. A game with unlimited slots would have been forgettable. A game with eleven slots is a cultural artifact.

When you are designing your progression system and you are tempted to add more — more nodes, more stats, more slots, more options — remember Team Cherry's eleven notches. Sometimes the answer is not more. Sometimes the answer is tighter. And tighter, done well, is the strongest answer available.

Play Hollow Knight once without consulting guides. Equip different charm loadouts for each region. Note which combinations feel right to you, which feel wrong, and which feel revelatory. Then open the guide, read a dozen community-favorite loadouts, and try them. You will learn something about progression design no chapter can teach directly.