Insert a digit from 1 to 5 into every cell, so that each digit appears once in each row and column . Each digit in a cell represents the height of a building. Numbers outside the grid indicate how many buildings a distant observer could see (at least part of) in that row or column. A building will hide any shorter buildings behind it.
Grid control: Click at a cell reveals the value chooser. Click at the selected value changes the state of the cell. More details about grid control are at the help page.