Observer Killer Sudoku: Guide to Godefridus’s Weekly Puzzle

Observer Killer Sudoku is the weekly killer sudoku puzzle published by The Observer, the UK Sunday newspaper, under the byline "Killer by Godefridus". It follows standard killer sudoku rules — cage sums plus normal sudoku constraints — and is available in the paper's puzzles section with downloadable PDF versions.

What Is Observer Killer Sudoku?

The Observer, Britain's Sunday newspaper, runs a regular puzzles section that includes several sudoku variants — among them a weekly killer sudoku credited to the setter Godefridus, published under the heading "Killer by Godefridus" (observer.co.uk).

Like all killer sudokus, it takes the familiar 9×9 sudoku grid and overlays it with "cages" — irregular shapes outlined in grey — each carrying a small target number. Your job is to fill the grid so that every row, column and 3×3 box contains the digits 1–9 exactly once, while the digits inside each cage add up to that cage's target number.

The Observer's puzzle pages follow a dated format — for example, "Killer by Godefridus (16 November)" — and sit alongside the paper's other weekly puzzles such as Sudoku Classic and Big Apple sudoku (observer.co.uk).

The Observer's Killer Sudoku Rules

The paper prints its rules with every puzzle, and they are refreshingly short:

> Normal Sudoku rules apply, except the numbers in the cells contained within grey lines add up to the figures in the corner. No number can be repeated within each shape formed by the grey lines. (The Observer's rules)

That is the whole game: standard sudoku plus cage arithmetic. If you can already solve a normal sudoku, you only need to learn the cage logic — and if you are new to the classic game, start with our guide on how to solve sudoku before taking on the killer.

Where to Find the Observer Killer Sudoku Each Week

• Observer.co.uk puzzles section. Each week's "Killer by Godefridus" puzzle is published online in the paper's puzzles area, dated by the Sunday it appears (observer.co.uk).
• Downloadable PDF. The Observer also releases the puzzle as a printable PDF — handy if you prefer solving on paper with a pencil (cdn.observer.co.uk).

Because it is tied to the Sunday paper, a new puzzle appears weekly rather than daily — a nice slow-burn challenge compared with the daily killers on puzzle apps.

Killer Sudoku Basics: Cages and the 45 Rule

Before tackling Godefridus's grid, make sure the core concepts are solid. Our full explainer What Is Killer Sudoku covers the variant in depth; here are the essentials:

• Cages are the grey-outlined shapes. The small number in a cage's corner is the sum its digits must total.
• No repeats inside a cage — even if a cage spans two rows, its digits must all be different.
• The 45 rule: every row, column and 3×3 box contains the digits 1–9 exactly once, so each always sums to 45. This is your most powerful tool: if the cages inside one box sum to, say, 52, the cells spilling outside the box must total 7 — which often reveals a digit outright.

Tips for Solving the Observer's Killer

1. Start with the extreme cages

Cages with very small or very large sums have few possible digit combinations. A two-cell cage summing to 3 can only be {1,2}; a two-cell cage summing to 17 can only be {8,9}. Lock these in first.

2. Pencil in cage combinations

For each cage, jot the possible digit sets in the margin. Many combinations are unique — a three-cell cage summing to 6 must be {1,2,3} — and knowing the set narrows every cell it touches.

3. Apply the 45 rule to every region

Scan each row, column and box: add up the cage totals fully inside it, compare with 45, and deduce what the leftover cells must be. This "innie/outie" logic cracks open most killer puzzles.

4. Cross-hatch like a normal sudoku

Do not forget ordinary sudoku technique. Cage sums get the glory, but plenty of Observer killer cells fall to simple row/column/box elimination once a few digits are placed.

5. Treat it as a Sunday puzzle

Godefridus's killers are designed for a leisurely Sunday solve — they reward patience over speed. Work in pencil, revisit stuck regions after a tea break, and let the cage arithmetic do the heavy lifting.

Observer Puzzles Compared

All three appear in The Observer's puzzles section, so Sunday solvers can warm up on the classic before taking on the killer (observer.co.uk).

For more number-puzzle reading, our guide to sudoku puzzle books rounds up the best print collections if you want killer sudokus beyond the Sunday paper.

Paper or Screen: How Most Readers Solve It

Because the puzzle ships as a printable PDF, many solvers work it in pencil on paper — cage combinations jotted in the margins, the 45 rule worked out longhand. That slower pace suits a Sunday puzzle. If you prefer digital solving, a killer sudoku app lets you enter the grid and manage pencil marks automatically. Either way, the puzzle is the same; pick the medium you find more relaxing. For more print-friendly puzzling, see our sudoku puzzle books roundup.

Building a Weekly Killer Routine Around the Observer's Puzzle

A weekly newspaper killer is a different discipline from app-based solving, and treating it that way will improve both your enjoyment and your success rate. Here is how experienced paper solvers approach the Observer's Sunday killer.

Solve it on paper first, even if you read the paper online. The Observer's puzzle is designed for print — the cage outlines, the generous grid, the lack of a timer. Printing the PDF or solving in the paper keeps you in the setter's intended medium, and many solvers find that pencil-and-paper forces better habits: you write full candidate lists instead of relying on auto-notes, which builds the pattern recognition that apps can dull.

Give it the Sunday slot it deserves. Godefridus's killer is pitched as a weekend puzzle — meatier than a daily but fair. Set aside 30 to 45 uninterrupted minutes rather than squeezing it between tasks. Killer sudoku rewards sustained attention: the 45-rule calculations and cage-combination checks need working memory that constant interruption destroys.

Work the extremes before the middle. As the article's tips note, extreme cage sums (very small or very large) have the fewest possible combinations. On a weekly paper killer, which tends to be carefully constructed, those extreme cages are deliberately placed as your entry points — the setter wants you to find them. Start every solve by circling the smallest and largest cage totals on the grid.

Keep a "killer notebook". Because the puzzle appears weekly, you can track your solving times and the techniques each puzzle required. Over a few months you will see exactly which skills — innies and outies, locked sets, the 45 rule — need work, which is far more useful than any app's generic statistics.