68 challenges across eight tiers. Tier 8 is a warm-up; tier 1 will take an evening. Nothing is locked — play them in any order.
The boss. Is the black king attacked by anything at all — pawn, knight, bishop, rook, queen or the white king?
Extract every white bishop that is giving check along a diagonal, in board order.
Does a white rook attack the black king along a rank or a file? Any piece standing between them blocks it.