Monday Mutant 25: Melon's Secret Castle

Draw a single loop that connects the grid cells. The loop may only travel horizontally or vertically, and never diagonally (so all turns are of 90 degrees). The loop may only turn at the centers of grid cells, and may not cross itself or branch off. A cell containing a number and an arrow represents the total length of the horizontal (for a left or right arrow) or the vertical (for an up or down arrow) line segments in the row or column pointed at by the arrow (or, equivalently, how many cell boundaries the loop passes through in that direction). The loop may not pass through a cell containing a numbered arrow clue, but all such clues which are in a white cell must be inside the loop, and all black cells must be outside the loop.
As my oh-so-clever title alludes to, this puzzle genre was invented by MellowMelon.