Light Up (Akari): the rules, and how to play

The classic rules

In the classic puzzle, some walls carry a number from 0 to 4. The number says how many lights stand on the squares directly above, below, left and right of that wall. A wall without a number says nothing.

1. Place lights on white squares.
2. A light shines along its row and column until it reaches a wall or the edge of the grid.
3. No light may shine on another light.
4. A numbered wall has exactly that many lights on the squares that share a side with it.
5. Every white square must be lit.

The rules in Nightwatch

Nightwatch plays a variant called Regional Akari. The walls carry no numbers. Instead the open squares are divided into coloured courtyards, and the courtyards are the clues:

1. Every courtyard gets exactly one lantern.
2. A lantern lights its own square and shines along its row and column until a wall stops it. A courtyard edge does not stop light, and a lantern set on a lit square is dazzled, which is never allowed.
3. No lantern may shine on another lantern.
4. Every open square must end up lit.

The edge of a courtyard is not a wall. Light passes straight through it and stops only at a wall, which is what makes the variant interesting: a lantern in one courtyard can rule out squares in several others. Regional Akari, explained.

Try it

This is the first round of the Nightwatch campaign, Lamplighter Steps: five squares across and six courtyards. Tap a square to stand a lantern, tap it again to take it away, and hold a square to cross it out.

The same board, solved step by step

Every move below follows from what is already on the board; nothing is guessed. The sentences are the ones the game's own solver writes.

1. Step 1 Only one square is left in this courtyard, so its lantern goes at row 5, column 1.

2. Step 2 The lantern at row 5, column 1 already shines through row 5, column 2, so no lantern can stand there.

3. Step 3 Only one square is left in this courtyard, so its lantern goes at row 4, column 2.

4. Step 4 The lantern at row 4, column 2 already shines through row 2, column 2, so no lantern can stand there.

5. Step 5 The lantern at row 4, column 2 already shines through row 3, column 2, so no lantern can stand there.

6. Step 6 The lantern at row 4, column 2 already shines through row 4, column 3, so no lantern can stand there.

7. Step 7 The lantern at row 4, column 2 already shines through row 4, column 4, so no lantern can stand there.

8. Step 8 Only one square is left in this courtyard, so its lantern goes at row 3, column 4.

9. Step 9 The lantern at row 3, column 4 already shines through row 1, column 4, so no lantern can stand there.

10. Step 10 The dark square at row 1, column 5 can only be lit by a lantern standing on it.

11. Step 11 The dark square at row 2, column 3 can only be lit from row 2, column 1, so the lantern belongs there.

12. Step 12 The lantern at row 3, column 4 already shines through row 3, column 5, so no lantern can stand there.

13. Step 13 The lantern at row 4, column 2 already shines through row 4, column 5, so no lantern can stand there.

14. Step 14 Only one square is left in this courtyard, so its lantern goes at row 5, column 5.

Three things to notice:

• It starts with a courtyard that is a single square. Rule 1 says that courtyard needs a lantern, so the lantern goes there.
• Most of the work is crossing squares out. A lantern lights its whole row and column up to the walls, and no other lantern may stand on a lit square.
• Near the end, a dark square can be lit from only one place, so that place holds a lantern.

Where the names come from

Light Up is the English name. Akari is a Japanese word for a light, and in Japan the puzzle is also known as 美術館 (bijutsukan), which means art museum. Regional Akari, the variant Nightwatch plays, keeps the light and replaces the numbered walls with courtyards.

Light Up (puzzle), Wikipedia