1. Place a number from 1-N into each cell, where N is the length of the grid.
Each number appears exactly once in each row and column. Numbers inside the
grid represent the height of a building.

2. Each number outside the grid represents the number of buildings an observer
sees from that location. Buildings are opaque and can block buildings behind them.

3. If an observer number is not in a square, it is a standard skyscraper clue,
i.e. a building hides any building of smaller height behind it from that observer.

4. If an observer number is in a square, it represents the number of visible
buildings seen assuming the observer is standing 1 cell away from the grid.
(See the example.)