What does each level ask of the solver?
- Easy: every number can be found by looking along one row, column or box at a time.
- Medium: now and then a square has only one number left that fits, or a number is locked into one line of a box and can be crossed off the rest of that line.
- Hard: pairs and triples (two or three squares that share the same few numbers) and the X-Wing, a pattern across two rows.
- Expert: patterns that span the grid: the Swordfish, the wings and linked pairs of one number.
- Evil: chains, following a number or pairs of numbers from square to square to see what cannot be true. Killer levels are described in cage terms: Easy uses the few sets of numbers a cage total allows, Medium adds the 45 rule, Hard uses the 45 rule on groups of squares with pairs and triples.