Killer Cages, Explained: The Sudoku Variant That's Secretly Just Algebra

Killer mode in Grandmaster Sudoku looks intimidating for one reason: almost none of the starting numbers are there. Instead, the grid is carved into dashed-outline “cages,” each with a small sum printed in the corner. Your job is to fill every cage so its cells add up to that number — while still following normal sudoku rules for rows, columns, and boxes.

The trick is that a cage sum quietly rules out most combinations before you’ve placed a single digit. A two-cell cage summing to 3 can only be {1,2} — there’s no other way to make 3 from two different digits 1–9. A two-cell cage summing to 17 can only be {8,9}. The smaller the cage, the more it behaves like a tiny algebra problem with a unique (or near-unique) answer.

A few starting points that make Killer readable instead of overwhelming:

• Two-cell cages first. Sums of 3, 4, 16, or 17 only have one possible pair. Find those before anything else.
• Watch the 45 rule. Every row, column, and 3×3 box still sums to 45. If a box is covered by cages that mostly stay inside it, the leftover cell’s value is just 45 minus everything you already know.
• Cages don’t repeat digits. A cage can never contain the same number twice, even if it spans multiple boxes — that alone eliminates a lot of combinations fast.

Once a couple of two-cell cages fall, the rest of the grid tends to open up the way a normal sudoku does — you’re just doing a little more arithmetic to get there.