Fill in the grid with towers whose heights range from 1 to the grid size, so that every possible height appears exactly once in each row and column, and so that each clue around the edge counts the number of towers that are 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 the right, it would match a clue of 1 because the 5 hides everything else.)
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.