Each square holds a building of height 1 to N, one of each per row and column. The numbers around the edge say how many buildings you can see looking down that line — a taller building hides every shorter one behind it.

• A clue of 1 means the tallest building is right there in front.
• A clue of N forces the whole line into order: 1, 2, 3, …
• A clue of 2 in a 4×4 rules out 4 in the first square.

Solved clues fade out. This one trains spatial reasoning and constraint chaining at once.