Nobody looks at a level and decides whether it is hard. Difficulty is measured, and every level has to sit inside its act's band — if it does not, it does not get into the repository.
Four metrics
Every level is solved end to end by the game's own solver, producing four numbers:
- Minimum move count. This is the level's
par, and it is never written by hand. - State space. How many distinct reachable states the level has; the real size of the puzzle.
- Dead-end rate. The share of states from which the solution is already lost.
- Solution variety. How many distinct solutions exist within minimum + 2 moves.
The bands
Every act sits in a band for each of those four numbers:
| Act | Minimum moves | State space | Dead ends | Solution variety |
|---|---|---|---|---|
| I | 6–14 | < 5,000 | ≤ %0 | ≥ 3 |
| II | 10–22 | < 50,000 | ≤ %10 | ≥ 2 |
| III | 14–30 | < 400,000 | ≤ %20 | ≥ 1 |
This table is the one the level validator enforces in CI — the band shown to the reader and the band checked by the machine cannot be two different things.
You can see where the levels actually land in the table on Acts; those values come from the catalogue too.
Gates alongside the bands
A band says how big a level is; it does not say whether it is any good. So alongside the bands the validator also checks the quality of the level. These are the gates:
- Every level has at least one enemy.
- At least one of the shortest paths drawn as if the enemies were not there is lethal because of them, and the shortest solution contains at least one interaction: a takedown, a stone or a detour to avoid something. A level that can be finished ignoring the enemies does not pass the gate.
- Every enemy does a job in the level: remove it and the shortest solution or the solution set changes. There are no enemies placed for decoration.
- Tutorial levels cannot be passed without the rule: no solution that avoids the rule is found within three moves of the shortest solution.
- At most thirty percent of a level's edges are one-way.
- In levels with caravan members, every member has at least one solution that is impossible without another member's action.
- The third star must be reachable in every level and must offer a route that is longer than par but bounded.
The jump between consecutive levels is measured too: if the increase in par between two consecutive main levels of the same act exceeds two moves, or the growth of the solver's state space exceeds two and a half times, the validator warns. That is a warning, not an error — deliberate jumps can be accepted with a justification.
Why the dead-end rate opens slowly
A dead end is losing the solution without noticing: opening a door at the wrong moment, spending coins on the wrong gate, walking a member into a corner they cannot be recovered from.
In act 1 that rate is near zero, because the player is still learning the rules. By act 3 it reaches 20% — but even there it costs one tap, because undo is unlimited. A dead end does not punish the player; it invites them to pay attention.
Why solution variety shrinks
Act 1 asks for at least three distinct near-minimum solutions: a new player has to be able to find their own way through. By act 3 one is enough, because what is being asked for is no longer "a route" but the route.
That is also the explanation for why the Master star gets harder in act 3.