At 15 × 15 the board stops being one puzzle and becomes several: 225 cells resolve into regions that unlock in stages, each solved section tightening the boundaries of the next. Bookkeeping matters as much as logic — most wrong turns on this size come from committing a rectangle that merely fits rather than one that's forced.
The wide-open middle is where these boards earn the expert label. Away from the walls, a 12 or an 18 can have a dozen candidate rectangles, and progress hinges on area parity: what's left of a region after the forced pieces leave must still be coverable by the clues that remain inside it.
Build up through 10 × 10 boards first if this size stalls you, or browse easy 15 × 15s to acclimate to the scale before taking on the harder tiers.