Star Battle
How to solve Star Battle: five moves, no guessing
Updated Tuesday, October 27
To solve a Star Battle, place the stars that have no choice, then dot every square around each star: with a dot in every square a star cannot use, each line soon has exactly as many free squares as stars to place. Every Gridarium star battle can be finished this way, without guessing.
The rules
Thick lines split the grid into as many regions as it has rows. Place stars so that every row, every column and every region holds the same number of stars: one on the smaller grids, two on the largest. Two stars never touch, not even at a corner. That last rule does most of the work: a star empties the eight squares around it.
Step 1: dot the neighbours
Every time you place a star, mark the eight squares around it with a dot: no star can go there. Dots are the pencil marks of this game. A square whose row, column or region already has all its stars gets a dot too.
Step 2: lines that are full
When a row, a column or a region holds all its stars, every other square in it is a dot. When the squares still free in a line are exactly as many as the stars it still needs, they are all stars. Small regions give this away first: a region of two or three squares with one star to place rarely has a choice.
Step 3: squares a star cannot avoid
Look at where a region's remaining star could still go. If every one of those squares touches the same outside square, that square can never hold a star: dot it. The same works along a row: two free squares side by side must hold the row's last star, so the squares above and below both of them are dots.
Step 4: count regions against rows
Two rows hold exactly two stars on a one-star grid (four on a two-star grid). If two whole regions fit inside those two rows, they use up all of the rows' stars: every other square of those two rows is a dot. Turn it around as well: if a region spills across two rows that are otherwise full, its star must sit in the spill. This is the move the medium and hard grids ask for.
Step 5: one careful trial
When nothing else moves, pick a free square, imagine a star there and dot its neighbours. If a row, column or region is left with fewer free squares than stars to place, the star was impossible: that square is a dot. One step is always enough on Gridarium; you never have to guess twice.
The three levels
- Easy: 6 by 6, one star per row, column and region. Steps 1 to 3 are enough.
- Medium: 8 by 8, one star per line: the classic size. Step 4 is usually needed.
- Hard: 10 by 10, 2 stars per line, where step 4 comes up again and again and step 5 sometimes.