The goal of Nullum is to eliminate all tiles by creating matching number pairs.

A tile can only be moved onto an adjacent (horizontal or vertical direction) tile.

When moved, if both tiles have the same number they both disappear from the puzzle. Otherwise, the numbers on them are added together to form a single bigger tile.

Levels 8, 9, 32, 36, 37 uses the "select a tile" mechanic; where you need to place the additional tiles (the ones with only a black border) into the empty blue square spaces in the puzzle before starting.

Levels 13, 14, 19, 23 uses the "question mark tile" mechanic; where you can select any single digit number (1-9) you want for tile(s) with a question mark on it before starting to solve the puzzle. The number is picked by right clicking on the ? tile until you reach the number you want; clicking when the tile has a 9 will reset the value to 1; once you start the puzzle the value can't be changed.

Check out my guides to other puzzle games here.