How to solve Light Up: six deductions, step by step

Where to start

Read the board before you place anything. A courtyard of a single square is a free lantern: rule 1 says each courtyard holds exactly one, so that square holds it. After each lantern, cross out every square it lights. A crossed-out square can never hold a lantern, and the crosses are what make the next move visible. The first three deductions below come up on most boards; the last three are needed when those run out.

1. The last square

When a courtyard has only one square left that could hold a lantern, the lantern goes there. At the start of a board this is a courtyard of one square. Later it is a courtyard whose other squares have all been crossed out.

Every campaign board and every nightly board in Nightwatch needs this at least once.

2. Already lit

A lantern's light runs along its row and column to the next wall. No square in that light can hold a lantern, because a lantern there would shine on the first one. Cross those squares out straight away.

You will use this on every campaign and nightly board. It does not place a lantern by itself, but it is what turns a courtyard of three or four open squares into a courtyard of one.

3. The only light

Every square must end up lit. Pick a dark square and ask which squares could still light it: the squares of its own row and column up to the walls, itself included, that are not crossed out. If only one is left, a lantern must stand there.

Sometimes the only square that can light a dark square is the dark square itself. Then the lantern goes on it.

4. One courtyard reaches it

Take a dark square and list every square that could still light it. If all of them belong to the same courtyard, that courtyard's one lantern must be among them, so every other square of that courtyard can be crossed out.

This is the first deduction that works on a courtyard from the outside: the dark square does not have to be in that courtyard at all.

5. It would leave a square dark

Before you place a lantern, look at what it would cross out. If a lantern on some square would cross out every square that could light some other dark square, that dark square could never be lit. So the lantern cannot go there.

This one asks you to look one move ahead. It is needed on 48.6% of the campaign boards and 42.7% of the nightly ones.

6. It would box in a neighbour

The hardest deduction looks at two courtyards at once. If a lantern on some square would light every square a neighbouring courtyard still has, that neighbour would be left with nowhere to stand its own lantern. So the square is out.

It is needed on 27.8% of the nightly boards. In the campaign it appears only in the last two districts, Old Town and The Long Night.

Classic Light Up: three quick starts

If you play the classic puzzle with numbered walls, the same habits apply, plus three openings:

• A wall marked 4 has a light on all four sides. Place them first.
• A wall marked 0 has no light beside it. Cross out its neighbours.
• When a number equals the open squares around its wall, every one of them holds a light: a 3 against the edge of the grid, or a 2 in a corner.

Using the hints in Nightwatch

Nightwatch's hints are built on these deductions. The first hint names the technique the board is asking for, the second shows the squares it acts on, and neither places a lamp for you. A third can point to one square that has to hold a light and give you the deduction that proves it.