Hard boards use fewer, larger clues placed deep inside their rectangles, keeping many candidates alive at once. Solving them means chaining deductions: common cells claim territory, claimed territory strands cells, and stranded cells — orphans only one rectangle can still reach — force the next placement.
Every hard board is still verified to a single solution and solvable by pure deduction; none of our puzzles require bifurcation ("try it and see"). When you stall, hunt for the almost-enclosed empty cell: orphan prevention is the technique this tier is built around.
Hard is most at home on 10 × 10 and up — the 15 × 15 and 20 × 20 giants at this tier are the longest solves on the site.