How to solve a skyscraper in Sudoku
A skyscraper is a digit with exactly two places in each of two rows (or columns), lined up on one side, which clears it from cells that see both roofs.
How it works
Take two rows where a digit has only two places each. If one place in each row sits in the same column, those two cells form the base of the skyscraper, and the two other places are its roofs.
The base cells see each other, so they cannot both hold the digit. That means one of the roofs does: if one roof is not the digit, its base cell is, which rules out the other base cell, so the other roof must be. Any cell that sees both roofs cannot hold the digit. The same works with rows and columns swapped.
How to spot it
Pick a digit and look for two rows with exactly two spots each, where one spot in each row lines up in a column. The roofs, on the other side, are in different columns.
A worked example
This is a step from a real puzzle, a moment where the next move needs a skyscraper.
F are the base cells; R are the two roofs, one of which holds the digit. Struck-through digits are the ones that can be removed.
5 has only two places in each of two rows: R1C2, R1C9 and R9C9, R9C3. R1C9 and R9C9 share column 9, so they can't both be 5. That leaves R1C2 or R9C3 holding it: if R1C2 isn't 5, then R1C9 is, which rules out R9C9, so R9C3 must be. Remove 5 from cells that see both roofs: R7C2, R8C2, R2C3.
Where it fits in Rocksled Sudoku
Puzzles that need a skyscraper are never rated below Very Hard. Each step of this kind adds 36 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.