THE SHORT VERSION
Sudoku: place the digits 1–9 so that each row, each column, and each 3×3 box contains every digit exactly once. No clues outside the grid; given digits inside constrain what goes where.
Skyscraper: the same Latin-square constraint applies to the whole grid. In addition, numbers around the border tell you how many "buildings" of increasing height you would see looking inward along that row or column. A taller building blocks every shorter building behind it.
Same base grid. One extra rule. Significantly more reasoning.
SIDE BY SIDE
Base constraint
Sudoku: 1–9 once per row, column, and box
Skyscraper: same
Extra clues
Sudoku: given digits inside the grid only
Skyscraper: edge view-count numbers outside the grid
What a clue of "1" means
Sudoku: n/a
Skyscraper: the first cell from that edge must hold the maximum digit (it blocks everything)
What a clue of N means
Sudoku: n/a
Skyscraper: the row/column must be strictly increasing — every digit is taller than the last
Primary elimination technique
Sudoku: naked/hidden singles and pairs within rows, columns, boxes
Skyscraper: same, plus visibility enumeration from each border clue
Relative difficulty
Sudoku: baseline
Skyscraper: harder at the same grid size
WHAT STAYS THE SAME
Every technique you use in Sudoku applies unchanged to the inner grid. Naked singles (only one digit can go in this cell), hidden singles (only one cell in this row can hold this digit), naked pairs, pointing pairs — all work identically because the Latin-square constraint is identical.
If the given digits already fully constrain the puzzle, Skyscraper reduces to Sudoku and the edge clues are redundant. In practice the grid is under-determined without them; the edge clues are what makes the puzzle uniquely solvable.
Box elimination — ruling out a digit in a box because it must appear in a specific row or column of that box — works exactly the same. Nothing about the visibility layer changes how boxes interact.
THE VISIBILITY LAYER
Imagine standing outside the grid, looking along a row or column. Each cell holds a building of height equal to its digit. A building is visible if it is taller than every building between it and you. The edge clue is the count of visible buildings.
A clue of 1 means the tallest possible digit must occupy the first cell — it is the only building you see because it blocks all others. For a 6×6 grid the first cell must hold 6; for a 9×9 grid it must hold 9.
A clue equal to the grid size (6 for a 6×6, 9 for a 9×9) means the row or column must be in strictly ascending order from that edge — every cell is a new personal maximum, so every building is visible.
Most clues fall between these extremes and constrain which permutations of the remaining digits are valid from that direction. The key technique is enumerating all valid orderings for a given clue and finding digits that appear in the same position across every valid ordering — those placements are forced.
WHICH IS HARDER?
Skyscraper is harder because the solving loop runs in two directions simultaneously. A naked single in a row eliminates a candidate, which changes what orderings are valid for the edge clues of that row and its column, which may force more placements, which cycle back to more naked/hidden singles. The two constraint systems amplify each other — but only if you keep both active in your reasoning at once.
Solvers who compartmentalise — "I'll do Sudoku logic first, then visibility" — stall on medium-difficulty Skyscraper. The puzzles are designed so that neither layer alone can make progress past a certain point; forward movement requires feeding inferences from one layer into the other.
WHEN TO MAKE THE SWITCH
If you play Sudoku: try Skyscraper when you can solve a standard 6×6 or 9×9 quickly without writing pencil marks. Start with a 6×6 Skyscraper — the smaller grid means fewer permutations to consider per edge clue, so the visibility layer is learnable without being overwhelming.
If you play Skyscraper: try Sudoku when you want the pure Latin-square workout without visibility overhead. It is a good warm-up before tackling harder Skyscraper tiers — your pattern recognition for row and column interactions gets sharper.
Playing both builds two complementary reasoning modes: pure elimination-within-groups (Sudoku) and constraint-layering across the two systems (Skyscraper). Skyscraper players tend to become faster at Sudoku because the extra pressure trains faster naked-single recognition.