Logic Masters Deutschland e.V.

Skyscraper Sudoku — "Inner Sky"

First, the actual puzzle:

Penpa version (thanks, Vebby!)

And now the directions: normal sudoku rules apply. If you imagine the digits as skyscrapers with corresponding height, then the clues along the edges of the grid indicate how many skyscrapers one can see from this direction (smaller buildings are obscured by larger ones). So there is a skyscraper with 4 floors in row 4, column 7.

The nine cells of row 3 gaze down at their respective columns, rows 4-9, and if their digit would be a valid skyscraper clue for those 6 cells, then a little arrowhead points in that direction (▼). If no arrowhead is present, it means the clue would be incorrect.

The same is true for the cells of row 7, gazing up their columns at cells in rows 6-1. You'll always see an arrowhead pointing upward (▲) if the digit would be a valid clue.

...and also for column 3 looking to the right at columns 4-9 (►), and for column 7 looking left at the columns 6-1 (◄).

PLEASE NOTE:The very same principle applies in the cells in row 4 gazing up at rows 1-3, row 6 down, column 4 leftwards, and column 6 rightwards....but in all these thirty-six cases there were no arrowheads to be entered!

To put it another way, the search for skyscraper values only takes place across the thick borders between boxes. Here's an example, where all valid skyscraper clues are marked and invalid ones left unmarked: