How to play
Click or tap a hidden cell to put a dot on it: safe. Click it again to clear it. Right-click, long-press or switch on Flag to place a flag. The counter shows how many mines are still without a flag. On a keyboard: the arrows move, Space or Enter puts a dot, F a flag.
Check looks at every mark at once. One wrong flag or dot is enough for Boom!, and then the board starts again from scratch. What was wrong it does not say.
About this puzzle
A 9×9 board with 15 mines and 21 numbers given. Very hard: more than half of the moves need three numbers or more, and the first easy one comes late.
Why the name: Novenarius is a Latin word meaning “containing 9”, after the 9 columns of the board, and XXI is 21 in Roman numerals: how many numbers you are given.
The steps these puzzles need are explained in the lessons Three numbers together, The whole border at once and Counting mines.