Here is a link to what are called skyscrapper puzzles. The puzzle consists of a rectangular grid of dimensions n x n. Each row and column is to be filled with numbers from 1 to n so that all numbers occur in each row/column exactly once. These numbers represent building heights. Another constraint is the number of buildings that can be seen from the beginning/end of each row/column. These constraints are written by the side. The first few levels should be easy, but levels 22 and 23 are very hard.