Difficulties

Ten difficulty levels are available, ranging from lots of clues for the newcomer to puzzles that will challenge even experts. A puzzle's level is set by the hardest technique it needs, which the app finds by solving it. Each level may also need any of the easier techniques listed before it.

Quickie

Naked singles only (a cell with just one digit left), and lots of clues: a quick solve.

Easy

Naked singles only, with fewer clues.

Moderately Easy

Hidden singles: a digit that can go in only one cell of some row, column, or box. Hidden singles: a digit that can go in only one cell of some row, column, or group. Hidden singles: a digit that can go in only one cell of some row, column, or box (group, on an irregular board).

Moderate

Locked candidates and naked pairs.

Moderately Hard

Hidden pairs, and naked and hidden triples and quads.

Hard

The X-Wing, and double restriction (locked candidates spread across two rows or two columns at once).

Vicious

The Swordfish, and the turbot family: Skyscraper, 2-String Kite, and Turbot Fish.

Fiendish

XY-Wing, XYZ-Wing, W-Wing, Jellyfish, and X-Chain.

Hell

XY-Chain, almost locked sets (ALS), and Franken Fish.

Beyond Hell

Forcing nets, which follow where a candidate leads until it runs into a contradiction. The app's own puzzles never need more. An imported puzzle that can only be solved by guessing is filed here too, and its import preview says Solvable without guessing: no.

With the digit assistant on, a hint card names the technique the next step uses and explains how it works; with it off, a hint goes straight to the next digit you can place. The levels line up with the difficulty ladder at sudoku.coach, a good resource for learning the advanced techniques.