Fill in the grid with towers whose heights range from 1 to the grid size, so that every height appears exactly once in each row and column, and each clue around the edge counts the towers visible when looking into the grid from that direction. (Taller towers hide shorter ones behind them. So the sequence 2,1,4,3,5 would match a clue of 3 on the left, because the 1 is hidden behind the 2 and the 3 is hidden behind the 4.)
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click). A key the puzzle’s controls show as a button can be pressed there instead of on a keyboard.
To place a tower, click in a square to select it, then type the desired height on the keyboard. To erase a tower, click to select a square and then press Backspace.
Right-click in a square and then type a number to add or remove the number as a pencil mark, indicating tower heights that you think might go in that square.
Left-click on a clue to mark it as done (gray it out), and left-click it again to unmark it. Keep the left button down and drag to do the same to every clue you pass over that looked the same as the first.
Pencil mode is sticky: once a right-click switches it on, it stays on wherever you click, until you right-click again. The Marks key is lit while it is on, and pressing the key also switches it. To have it last only until your next click, turn off Right-click toggles a sticky pencil mode (stays on until right-clicked again) in the preferences.
You can also use the arrow keys to move the selected square around. Press Enter to toggle between entering heights and entering pencil marks, and Backspace or Space to clear a square. Shift or Ctrl with an arrow key marks the clue at that end of the selected square’s row or column as done.
Press Fill marks among the puzzle’s controls to write in every pencil mark wherever nothing is placed and no mark is written yet. The M key does the same, on any board where M is not something you type. Marks you have narrowed are left alone, and pressing again never starts over: see Filling in all the pencil marks. Once every empty square has marks, the press crosses out of each the heights already standing in its row and column.
Hint explains the next step rather than simply making it. It begins with any clue of 1 or of the full grid size, which place towers outright with no pencil marks needed, then fills in every cell’s pencil marks (the same as pressing Fill marks), crosses out the heights already standing in each row and column, and then works from your own marks, so it carries on from wherever you are. If a tower you’ve placed is wrong, or a cell’s pencil marks have crossed out its answer, it asks you to fix the highlighted mistakes first. If you would rather it left a cell unmarked until a step needs it, set Hints pencil in to Only as needed in the preferences: a cell with no marks then counts as holding every height not already in its row and column.
A clue is named by its number (“clue 3”); two outlined clues at either end of a line are a facing pair.
The hint speaks of heights and of what a clue sees, the towers visible from it. A few clue ideas are worth learning by name:
On the hardest boards that need no guessing, a step may be a wing. Take a cell whose pencil marks are down to two values, say a or b, and two cells it can’t repeat a value with, both outlined: one marked a and c, the other b and c. Whatever goes in the middle cell, one of the outlined cells must have c, so a cell that can’t repeat a value with either of them can’t be c. If the middle cell could be c as well, the same holds for a cell that can’t repeat a value with the middle cell either.
If you’ve turned on the preference that removes a height from pencil marks in its row and column when you place a tower, the hint leaves that tidying to the placement rather than spelling it out as a step of its own. On an Unreasonable board, deduction can run out before the grid is full; the hint says so rather than guessing.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.