Sentinels #109

• Region locked to a line. If every open cell of a region sits in one row, that region’s sentinel is somewhere in that row - so no other cell in the row can hold one. Cross the rest of the row out.
• Last cell in a region. Every coloured region holds exactly one sentinel. A region down to a single open cell is solved.
• Last cell in a column. Every column holds exactly one sentinel. When only one cell in a column survives, place the sentinel there.
• Last cell in a row. Every row holds exactly one sentinel. When a row has only one cell left that is not crossed out, that cell must be the sentinel.
• Neighbour squeeze. Sentinels never touch, even diagonally. When a row’s candidates are packed into a narrow band, any cell in the row above or below that would touch every one of those candidates can be crossed out.

Every one of these is explained properly in the Sentinels guide.