Six moves every island charter should know
The same handful of ideas solves almost every Number Islands puzzle. Learn to spot them and the hard levels start to fall.
The hints in Number Islands always explain why a move must be true. Behind those explanations sit a small number of reasoning patterns. Here are the six you'll use most, with the exact wording the app uses so you'll recognise them.
1. The sealed six
“Coastlines enclose exactly six cells here, so they must all be one island.”
Whenever coastlines wall off a pocket of exactly six squares, that pocket is a complete island. Every gap inside it is land, not sea. It's the most common move in the game and often the first thing to look for.
2. The full island
“These six cells are already bridged into a full island, so a coastline runs along its edge.”
The flip side of the sealed six. If bridges have already joined six squares into one island, nothing else can join it, so every edge around it is a coastline.
3. The only way out
“These 3 cells must grow into an island of six, and this is the only edge left open.”
A group smaller than six has to grow. If coastlines block every direction but one, the island must grow through that last gap. A single square boxed in on three sides is a classic beginner's version.
4. Same number, different islands
“Both sides already contain a 5, so they can't be the same island.”
An island holds each number only once. So if two neighbouring groups already contain the same number, the edge between them must be a coastline. This is where numbers start drawing the map for you.
5. Too big, or a trapped pocket
“A coastline here would wall off 8 cells, which can't be split into islands of six.”
Count before you draw. Joining two groups that would total more than six squares is impossible. Likewise, a coastline that cuts off a region whose size isn't a multiple of six (like 4, 8 or 10 squares) can't be right. These counting moves get more important in Ember Cliffs and The Far Storms.
6. The last number standing
“Its row, column and island already use every other digit, so only 4 fits.”
This is the sudoku move you already know. Look at a square's row, column and island together. If five different numbers are already used, the sixth goes here. Its twin is the hidden single: “In row 2, this is the only cell that can take a 2.” Pick a number, scan a row, column or island, and see where it can still go.
One catch: you can only use the island part of these number moves once you know where that island's coastlines are. Mapping and numbering go hand in hand.
Putting it together
A good rhythm is to sweep the board for sealed sixes and only-ways-out, then switch to numbers in any island you've finished, then use those numbers to find new coastlines. Repeat. When you get stuck, count the sizes of open regions. A trapped pocket is often hiding in plain sight.
To see every one of these moves in a real puzzle, read our step-by-step walkthrough of First Landfall.
