Castle Wall is a puzzle type originally created by Palmer Mebane
Draw a single closed loop in the grid. Squares with a thick border will not be a part of the loop; if they are white then they’ll be inside the loop, and if they are black then they’ll be outside. Some squares will have a number and an arrow in them. This indicates the number of boundaries in the arrow’s direction which are crossed by the loop.
See this example puzzle and its only solution.