Catdoku

Catdoku looks like a sudoku but contains no numbers at all. Each level is a square grid split into irregular coloured regions, and somewhere in it a set of cats needs a place to sit. The cats are fussy about personal space, and their three rules are the whole game: exactly one cat in every row, exactly one in every column, exactly one in every coloured region, and no two cats in neighbouring squares, diagonals included.

You work on the grid with taps. One tap puts an X in a square, your note to yourself that no cat can go there. A second tap on the same square swaps the X for a cat. A third clears it again. In practice most of a solve is X marks: the moment a cat goes down, its whole row, its column, its region and the ring of eight squares around it can be crossed out, and those crosses are what make the next placement obvious.

The usual way in is through the smallest region. A colour zone only two squares wide leaves two possible cat positions, and often one of them would touch something or share a line with another forced cat, so the answer falls out. Bigger boards demand a second technique: if two regions together sit entirely inside two rows, those rows are spoken for, and every other square in them can be ruled out. None of it needs a guess, and every level has a single solution.

The title screen offers numbered levels from one upward, a fish-shaped hint currency, a leaderboard under an automatically generated nickname, a daily challenge from level 21 and a Double Cats mode, with two cats per line, from level 30. Early grids are small enough to solve in under a minute, and the board size grows steadily as the level number climbs.