36 Cube

The puzzle consists of a gray base that resembles a city skyline, plus 36 colored towers.

The goal of the puzzle is to place all the towers onto the base so as to form a level cube with each of the six colors appearing once, and only once, in each row and column.

He came up with the idea while writing a book on whole numbers, after unearthing an 18th-century mathematical hypothesis.

Although Euler was correct, his conjecture was not settled until Gaston Tarry came up with an exhaustive proof in 1901.

Euler's 36 officer problem is a mental challenge, which can be attempted either in one's head or on paper, whereas the 36 cube is a mechanical puzzle that must be played on a manufactured grid.