Castle Wall is a logic puzzle genre developed by Palmer Mebane.
- Draw a loop through cell centers.
- The loop must not pass through shaded (including thick-bordered, which should be considered “shaded white”) cells.
- Shaded white cells must be inside the loop.
- Shaded black cells must be outside the loop.
- Shaded gray cells may be inside or outside the loop.
- If a shaded cell has a number and a direction on it, that number is equal to the number of segments along the line through the center of that cell in the given direction.