Star Battle › Guide
Star Battle Mistakes: Why No Valid Moves Remain
A board can have no immediate conflict and still be impossible to finish. Check the star quota, undo unsupported placements, and distinguish local rules from a full solution. No touching does not ban a whole diagonal, and a 2×2 block may contain zero stars.
Conflict highlighting detects direct rule violations such as touching pieces or too many stars in a unit. It is not a proof that the remaining cells can complete the board. A wrong move may remove the last possibility much later.
1. Banning a whole chess diagonal
A star at R1C1 excludes R2C2 by adjacency. It does not exclude R3C3 just because the two lie on the same long diagonal. Queens-style logic puzzles and Star Battle use immediate no-touch rules, not the full attacking reach of a chess queen.
This is a local illustration, not a full 4×4 solution. For all rule differences, read Queens vs Star Battle vs N-Queens.
2. Assuming every 2×2 block contains a star
Any two cells in a 2×2 touch, so the block holds at most one star. Exactly one requires an additional count. In the fully solved practice board below, the outlined block R1C4, R1C5, R2C4, R2C5 contains no stars at all.
Show a valid complete board with an empty 2×2 block
The row, column and region quotas still all hold. See when counting does force a star into a block.
3. Clearing a two-star unit after only one star
Two-star rows, columns and regions need two stars. After the first, exclude immediate neighbours; other distant cells in the unit may still hold its second star. Clear the unit's remaining cells only after its quota is met.
This is a teaching setup with one correct star from two-star practice #1, not a deduction of that star. The board still needs 19 others. Compare one-star and two-star quotas.
4. Treating a locally legal move as a forced move
Here is an intentionally wrong first star at R1C2 in a real unique 6×6 one-star puzzle. A single star cannot touch another star or exceed a one-star quota, so it passes those immediate checks. Nevertheless it is absent from the puzzle's only solution; no valid full board can extend it.
Play the original board without that wrong star. The problem is the unproved placement, not necessarily the last row where the contradiction becomes visible. Read how uniqueness is checked.
A short recovery routine
- Confirm one-star versus two-star rules.
- Find the earliest placement you cannot justify and undo it, including exclusions that depended on it.
- Return to a state supported by rules, then rescan every row, column and region.
- Use Queens checkpoints or the two-star walkthrough to practise proof before placement.