Most broken Skyscrapers grids come from one of six mistakes, and the biggest is placing a height before it’s certain. The others are misreadings of the clues: counting hidden buildings, reading a clue from the wrong side, treating a clue as a position, forgetting the first building always shows, and forgetting columns need each height once too. Each one is below, with how to spot it.
If you’re new, the rules and how to solve pages explain the ideas these mistakes go wrong on.
1. Placing a height before it’s certain
When a square could be one of two heights, it’s tempting to pick one and see. Often nothing tells you at all. We took every one of the game’s levels, put a wrong height in the first empty square (one that broke no rule yet), and kept making honest moves until the game showed a red mark:
| Track | Red mark appeared | Heights placed before it (median) | Longest |
|---|---|---|---|
| Easy 5×5 | 29 of 50 | 15 | 24 |
| Medium 6×6 | 30 of 50 | 20 | 33 |
| Hard 7×7 | 14 of 50 | 34 | 45 |
On the rest, the red mark never appeared: the honest moves ran out first, with the grid wrong. Here’s one on Medium level 3, with a wrong 1 in the top-left corner:
Twenty heights later, both of column 4’s clues turn red. Column 4 doesn’t contain the wrong height.
How to avoid it: only place a height when a technique makes it certain. If a square could still be two things, write both as notes.
2. Counting hidden buildings
A building only shows if it’s taller than everything in front of it. In 3 1 2, from the left you see only the 3: the 1 and the 2 are hidden behind it.
How to avoid it: walk along the line from the clue, and only count a building when it beats the tallest so far.
3. Reading a clue from the wrong side
Each clue looks inward from its own side. A row can show 2 from the left and 3 from the right.
How to avoid it: before using a clue, put your finger on it and read away from it.
4. Treating a clue as a position
A clue of 3 doesn’t mean the tallest building is third. It means three buildings show: 2 4 1 3 5 shows three from the left, with the tallest last.
How to avoid it: use a clue to rule heights out (distance from a clue), or to list orderings, never to put the tallest at that distance.
5. Forgetting the first building always shows
The nearest building is always seen, so the count starts at 1. A clue of 1 means the first building hides every other one: it’s the tallest.
How to avoid it: remember every clue counts the first building.
6. Repeating a height in a column
With clues to watch, it’s easy to forget the Sudoku part: each height once in every row and every column.
How to avoid it: after placing a height, check its row and its column, not just the clues.
How to recover from a mistake
- A red clue or height means a rule is broken, but the mistake may be elsewhere, often a long way back. Undo to the last move you were sure of.
- Press Stuck? It rings a wrong height, or a square whose notes leave out its height, before anything else.
- Stuck with no red mark? Check the clues again, then look at when you get stuck.