About the 15x15 preset
This is a starting point for the custom game builder: a medium 15×15 board carrying 35 mines across 225 cells, or 15.6% density. That puts it in the same difficulty band as Intermediate (40 mines on 16×16), so what changes here is scale rather than difficulty — 190 cells to clear instead of 216.
Tips for This Size
This smaller grid size is great for quick games and practicing patterns.