No 2×2 block can be all wall. A 2×2 block with three walls has its fourth square in an island.
This is one of the Nurikabe techniques. Each page shows its technique on the same 8×8 from the game, Medium level 8.
The rule
- Look for three walls in a 2×2 block.
- Dot the fourth square: it’s part of an island.
- Then ask which island it belongs to: that often settles more.
In a real puzzle
On the 8×8, it’s first needed at step 23. Row 1, columns 4 and 5 and row 2, column 5 are all wall. The fourth square of that block, row 2, column 4, must be island.
How often you’ll need it
On 49 of the 50 Easy levels and every Medium and Hard one: 3.8 times per Easy level on average, 7.4 on Medium and 12.1 on Hard. It’s 8 of the 42 steps on the 8×8 here.