Fill every open square with a ghost, a vampire or a zombie so that each sightline down the corridor counts exactly what it should.
The grid. Diagonal mirrors are already fixed in place. Every other square holds exactly one monster. The panel tells you how many of each kind live in the maze.
The numbers. Each number sits at the mouth of a corridor. Look straight in from there; the beam bounces off every mirror it meets and eventually leaves again. The number is how many monsters you would count along that whole route.
Who you can see. A vampire casts no reflection, so you only count it before the beam has hit any mirror. A ghost is invisible in the flesh but shows up in glass, so you only count it after at least one bounce. A zombie is always counted.
If a route passes through the same square twice, that monster gets counted twice — once for each pass, using that pass's rule.
Playing. Pick a monster, then click a square. Clicking the same monster again clears the square.
Turn on maybe marks (or right-click, or hold Shift) to pencil in two or three candidates while you work out which it is.
Keys: 1 2 3 4 pick a row, G V Z place one under the cursor, Space toggles maybe marks, Backspace clears a square.