Each cell in the N×N grid must contain a number from 1 to N. Think of each number as a building height.
Every row and every column must contain each number exactly once — like Sudoku.
Numbers around the grid edges tell you how many buildings are visible when looking from that side inward.
A taller building blocks all shorter buildings behind it. e.g. heights [2,1,3] → you see 2 (first), then 3 (tallest, blocks 1) = 2 visible.
A clue of 1 means the tallest building is first. A clue of N means buildings go 1,2,3…N in ascending order.