Rules: Draw a single closed loop (without intersections or crossings) passing through some empty cells in the grid. The grid contains some bordered or colored cells that cannot be part of the loop. Black cells must be outside the loop; white cells (with heavy borders) must be inside the loop. Numbers and arrows refer to the total sum of the lengths of loop segments in the given direction. (An equivalent way to understand these values is to count the number of cell borders crossed by the loop in that direction.)
Answer String: Enter the length in cells of the horizontal loop segments from left to right in the marked rows, starting at the top. If the loop only has vertical segments in the marked row, enter 0. Separate each row’s entry with a comma. This example has the key “2,21”.
(Brief) History of Castle Wall: Castle Wall is an original loop puzzle style created by Palmer Mebane in 2009; the first examples were posted on his blog Melon’s Puzzles.
History of this example: This Castle Wall was written by Thomas Snyder.
Design rules for contributors: A Grandmaster Castle Wall will have a unique solution that can be reached by logic alone. Generally, a Grandmaster Castle Wall should have clue symmetry, or an interesting visual/logical theme if there is no symmetry. Sizes from 10×10 and above are recommended (maximum aspect ratio of 2:1 if rectangular).