Doku is a popular logic-based puzzle game that involves filling a grid with numbers. The standard version is played on a 9x9 grid, which is divided into nine smaller 3x3 boxes. The objective is to fill the grid so that each row, column, and box contains all the numbers from 1 to 9 without repeating any numbers. Players typically start with some numbers already filled in, known as "clues." The challenge lies in deducing the placement of the remaining numbers using logic and reasoning. Doku puzzles vary in difficulty, making them suitable for both beginners and experienced players.