What is Castle Wall?
Castle Wall is a loop puzzle built around a single, surprisingly powerful idea: a wall has an inside and an outside, and every clue tells you something about which side of the wall it is on. You draw one closed loop through the centres of the cells - the wall itself - and the bordered clue cells are the towers and markers that the wall is built around. The loop never crosses or branches and never passes through a clue cell, so the clue cells sit in the landscape while the wall winds between them.
Each clue cell carries up to two pieces of information. Its colour says which side of the wall it belongs to: a white (outlined) cell must end up inside the loop, a black (filled) cell must end up outside it. Its number-and-arrow, when present, counts the loop: a horizontal arrow gives the total length of horizontal wall segments in that row in the arrow's direction, and a vertical arrow gives the total length of vertical segments in that column. Put the two kinds of clue together and the wall has exactly one legal route.
- Draw a single closed loop through cell centres - the castle wall.
- The loop never crosses, never branches, and never enters a clue cell.
- A white clue cell must finish inside the loop.
- A black clue cell must finish outside the loop.
- A number with an arrow counts loop segments in that row or column, in the arrow's direction.
- Exactly one loop satisfies every clue - no guessing required.
How to play Castle Wall online
Click or tap the gap between two neighbouring cells to lay a piece of the wall, and click it again to remove it. The small dots mark the lattice the loop travels along; the bold-bordered cells are the clues, and the loop simply flows around them - you will notice there is no place to draw a line into a clue cell, because the wall can never go there. Keep joining segments until they close into one loop that keeps every white clue inside, every black clue outside, and every arrow's count exact.
Check marks anything that already breaks a rule - a branch, an arrow whose count has overshot, a wall that has trapped a black cell inside or shut a white cell out - without giving the route away. Hint removes a stray segment first and only then adds a correct one, Undo steps backwards, Reset clears the board, and Solution lays down the full loop when you would rather study a finished wall than fight it.
- Tap between two cells to draw or erase a wall segment.
- The loop flows around clue cells; you cannot draw into one.
- Check highlights rule conflicts; it never spoils the route.
- Hint repairs your loop before it extends it.
- New puzzle builds a fresh board for the selected size and difficulty.
Reading the arrows: count the wall, not the cells
The arrow clues are the engine of Castle Wall, and the key is to read them precisely. A number with a right arrow counts the horizontal wall segments lying in that row to the right of the clue; a left arrow counts them to the left. A down arrow counts the vertical segments below the clue in its column, an up arrow the ones above. You are counting unit pieces of the loop that run parallel to the arrow - equivalently, the number of times the wall crosses the grid lines as it heads off in that direction.
Two values are gold. A zero means there is no wall at all in that direction: the whole ray is empty, which often seals a region shut and forces the loop to detour. A large number, close to the room available, means the wall runs almost solidly across that stretch, pinning long straights into place. Everything in between is a budget you can spend - and because the arrow only counts one orientation, a horizontal arrow says nothing directly about vertical segments crossing the same cells, which is exactly the subtlety that makes the clues combine so well.
- Horizontal arrow: counts horizontal segments in that row, in the arrow's direction.
- Vertical arrow: counts vertical segments in that column, in the arrow's direction.
- A 0 means that direction holds no wall at all - a powerful sealing clue.
- A large count forces near-solid straights and removes freedom fast.
- An arrow counts only its own orientation, never the perpendicular segments.
Inside and outside: the wall as a boundary
The colour clues turn the loop into a boundary you have to respect. Because the wall is a single closed curve, every cell that is not on it is unambiguously inside or outside, and you can test any cell with a simple parity check: trace a straight path from that cell out to the edge of the grid and count how many wall segments you cross. An odd number of crossings means you started inside; an even number means outside. White clues must land on the odd side, black clues on the even side.
This crossing count is a deduction tool, not just a definition. If a white cell and a black cell sit next to each other with nothing between them, the wall has to thread between them - one crossing - so a segment is forced right there. If two same-coloured cells are neighbours, no wall can separate them, which forbids segments. Border cells are especially talkative: a cell on the outer edge is outside unless the wall wraps around it, so a black cell on the border is usually free of nearby wall, while a white one demands the loop bulge out to enclose it.
- Any non-wall cell is inside or outside by the odd/even crossing rule.
- White = odd crossings (inside); black = even crossings (outside).
- Adjacent opposite colours force a wall segment between them.
- Adjacent same colours forbid a wall segment between them.
- Border black cells usually sit clear of the wall; border white cells make it bulge.
Where the loop cannot go
Half of Castle Wall is knowing where the wall is forbidden, and the clue cells hand you that for free. The loop never enters a clue cell, so every edge touching a clue is dead before you start - clue cells are pillars the wall must flow around. That alone fragments the grid into corridors, and corridors are where loops are easiest to pin: a one-cell-wide channel between two clue cells leaves the wall only one way through.
The single-loop bookkeeping then does the rest. Every cell the wall uses has exactly two connections, so the instant a cell shows two segments you can mentally seal its other sides. A cell with one segment is a live end that must continue. And you must never close a small loop early while clue counts elsewhere are still unmet - the finished wall is one loop, not two. Combine the forbidden edges around clues with these degree rules and large parts of the board resolve without ever looking at a number.
- Every edge touching a clue cell is forbidden - the wall flows around them.
- Clue cells fragment the grid into corridors that funnel the loop.
- Each used cell has exactly two wall connections - no branches, no dead ends.
- A cell with two segments seals its other two sides.
- Never close a separate small loop while any clue is still unsatisfied.
A modern classic from Palmer Mebane
Castle Wall is a relatively young puzzle with a clear author: it was invented by the American puzzlemaker Palmer Mebane, who introduced it on his blog in 2011 and used it in puzzle competitions soon after. Unlike the Nikoli loop classics that drifted into the world anonymously over decades, Castle Wall arrived fully formed, with a name, a rule set and a designer behind it - and it spread quickly through the online and championship puzzle scene because its central trick is so clean.
That trick is the marriage of two older ideas. The inside/outside colouring is the heart of shading puzzles and of any loop that divides a board into regions, while the directional number-and-arrow clue is borrowed from the counting tradition you see in Yajilin and the arrow puzzles around it. Castle Wall fuses them: one clue can simultaneously tell you which side of the wall a tower stands on and how much wall runs past it. Few puzzles pack two such different kinds of information into a single marked cell, and that density is exactly why solvers took to it.
Castle Wall vs Slitherlink, Yajilin and Masyu
Castle Wall belongs to the single-loop family, so the closed-loop bookkeeping it shares with Slitherlink, Yajilin, Masyu and Country Road will already feel familiar. The differences are in what the clues say. Slitherlink draws its loop on the edges between cells and counts how many sides of each numbered square are used; Castle Wall draws the loop through cell centres instead and counts segments along whole rows and columns. Masyu gives you only pearl-shaped colour clues with no numbers - local geometry rather than counting.
The closest cousin is Yajilin, which also uses numbered arrows that count along a line - but Yajilin's arrows count shaded cells while Castle Wall's count wall segments, and Yajilin has no inside/outside colouring at all. What makes Castle Wall distinctive in the whole family is that single fusion: a clue that is at once a region marker and a length gauge. If you enjoy Slitherlink's wall-building or Yajilin's directional counting, Castle Wall is the puzzle that asks you to do both at once, and it rewards the switch with deductions neither parent can make alone.
- Slitherlink: loop on cell edges; clues count used sides of a square.
- Masyu: colour pearls only, no numbers - local loop geometry.
- Yajilin: arrows count shaded cells; no inside/outside rule.
- Castle Wall: arrows count wall segments and colours fix inside/outside.
- Shared core: one closed loop, two connections per cell, no early closure.
Grid sizes and difficulty levels
The 6x6 boards are the place to learn the moves: the loop is short, the corridors between clue cells are obvious, and a single zero or a border colour usually breaks the board open. On 7x7 the wall stretches out, arrow counts start interacting across the grid, and you will meet positions where an inside/outside deduction in one corner is what unlocks an arrow on the far side. The 8x8 boards are full puzzles - longer walls, more clues working together, and stretches where you have to read colour and count in the same breath before a single segment becomes certain.
Difficulty changes how much the clues spell out. Easy boards keep plenty of arrows and walls, so each segment follows from a short, local deduction. Medium thins the arrows, leaving more of the wall to be pinned by inside/outside reasoning. Hard thins them further, so you lean longer on the boundary logic and the forbidden edges before the loop takes shape. Whatever you pick, the generator solves every board before serving it and keeps only the ones with a single solution - so a stuck position is never a broken puzzle, only a deduction you have not found yet.
- 6x6 - short walls and clear corridors for learning the rules.
- 7x7 - longer walls where colour and count start to interact.
- 8x8 - full boards with clues that must be read together.
- Easy keeps more arrows; medium and hard lean on inside/outside logic.
- Every puzzle is verified to have exactly one solution.