A "fill the board puzzle" is a logic game where you cover every open square on a grid exactly once by extending one continuous path from a fixed numbered start, with orthogonal moves and dark cells acting as permanent walls. The continuous-path variant belongs to a broader family of fill-the-board puzzles that includes block-fitting layouts, picture-fill grids like Nonogram, and one-stroke drawing challenges, but it is distinguished by a single, irreducible rule: every open cell must be visited exactly once and the cells you visit must form one unbroken line. In a typical fill-the-board puzzle the difficulty comes from the order of moves rather than piece geometry, because every cell you color in removes itself from the candidate list for the rest of the route. The Fill One Line Puzzle browser game is a concrete implementation of this idea: five original fixed grids, each with its own numbered start, that you solve back to back for an exact 1,000-point total. Read on for how the variant works, how each level is laid out, and how the input rules keep pointer, touch, and keyboard play in sync.
Fill the Board vs Block-Fit and Picture-Fill Variants
The phrase "fill the board puzzle" can mean different things depending on the genre, so it helps to separate the continuous-path version from its closest cousins before diving into the rules. A continuous-path fill such as Fill One Line Puzzle asks you to start at a numbered cell and trace a single unbroken line through every open cell exactly once. Cells cannot be revisited, and the order of moves changes the reachable area.
A picture-fill game such as Nonogram uses row and column number clues to mark individual cells black or white until a hidden picture emerges. Each cell is decided on its own, so the order of marking does not change the outcome, and there is no requirement that the marked cells connect.
A block-fit sliding puzzle such as Klotski places discrete rectangular pieces into a frame. Cells are reused as pieces slide, and the constraint is piece geometry, not cell visitation. The grid is "filled" by completing a target arrangement rather than visiting every cell once.
| Variant | What you fill | How you fill it | Visit rule |
|---|---|---|---|
| Continuous-path fill (Fill One Line Puzzle) | Every open cell on the grid | One connected line from a fixed numbered start | Each open cell visited exactly once |
| Picture-fill (Nonogram) | Each empty cell becomes marked or unmarked | Per-cell decisions driven by row and column clues | Marked or unmarked per clue, no path required |
| Block-fit sliding (Klotski) | The free space inside a frame | Slide fixed rectangles into a target layout | Cells are reused as pieces move |
The difference that matters most for strategy is the visit rule. In a continuous-path fill, once a cell is on your line it is gone forever, which is why the path order is itself the puzzle. In a picture-fill or block-fit game, decisions stay local and order rarely matters.
Why One Continuous Path Changes the Strategy
Most board puzzles reward a finished layout. A fill the board puzzle in the one-path sense rewards a finished route, which is a much harder object to construct because every choice has reachability consequences.
Three structural facts drive the difficulty of the variant:
- The start is fixed. You always begin at the numbered cell on the grid, so your first move is forced to be one of its open orthogonal neighbors. You cannot rotate or relocate the start, and you cannot start from a corner of your choosing.
- Visited cells are removed. The path counter and numbered step indicators show every cell you have used, and no used cell can become a stepping stone again. As the line grows, the unvisited area shrinks in ways that are not always obvious until several steps later.
- Blocked cells are walls. Dark squares never accept input, so the only way to move from one open region to another is through a corridor that has not yet been entered. Corridors created by obstacles are therefore precious routes that must be crossed at the right time.
Because of these rules, a move that looks harmless late in the route can strand a corner or split the remaining open area into two unreachable regions. The path is also not an Euler-style edge drawing, where vertices may be revisited as long as every edge is used. Here the cells themselves are what must be covered, and each cell appears on the path at most once. That distinction shows up in the on-screen numbered sequence: the step counter increases by exactly one for every added cell, never by zero, which is how you can tell a route is actually being drawn rather than redrawn.
How to Play Fill One Line Puzzle
Each level starts on a fixed numbered cell and ends when one continuous path has visited every open cell on that grid. Use the steps below for a clean run.
- Locate the numbered start. The fixed starting cell is marked with a number on every board; everything else is either open or blocked.
- Extend the path to an open neighbor. Click an orthogonally adjacent open cell, or press and hold while dragging across several adjacent cells to draw the route in one motion.
- Keep the line unbroken. Every next cell must share an edge with the current endpoint, must be open, and must not already be on the path. Diagonal jumps are rejected silently.
- Cover every open cell. Continue until the path has visited every open square. The on-screen counter matches the total open-cell count for the level when you are done.
- Press Check or Enter. Choose the Check button or press Enter to verify the route. A level clears only when every open cell is on the path.
- Clear all five grids. Repeat for the remaining four boards. Each cleared level adds exactly 200 points for a final total of exactly 1,000.
For full control reference and a deeper walkthrough, see the Fill One Line Puzzle browser game.
Five Levels, Five Original Grids, Exactly 1,000 Points
The five boards in a single run are not random rotations of one template. They are original fixed puzzles designed to escalate in a specific order so each new constraint is introduced in isolation before being combined.
- Level 1 is an open three-by-three introduction with no blocked cells, useful for learning the click-drag and arrow-key rhythm on a board where every move is obviously legal.
- Level 2 adds a single missing cell, introducing the first obstacle that the path must route around without revisiting.
- Level 3 introduces protected corners, where leaving a corner too early can make it impossible to revisit because every other neighbor is already used.
- Level 4 narrows the corridors so the order of moves through a bottleneck starts to drive the solution rather than the route geometry.
- Level 5 combines longer routes with mixed obstacles, testing whether the patterns from earlier boards still hold on a denser grid.
A separate depth-first search oracle is used during development to enumerate every legal orthogonal route for each board and to confirm that the declared solution visits every open cell exactly once from the fixed start. That independent check is stronger than running the production validator twice, because it does not share the production logic. From the player's perspective, the guarantees are simpler: every grid has at least one valid route, the start never moves, and a cleared level always awards exactly 200 points. Five cleared levels therefore always total exactly 1,000 points, regardless of which route you take through each board. For a closer look at the layout of the five grids, see the Fill All Squares Game guide.
Pointer, Touch, and Keyboard Controls
The control model is intentionally the same regardless of device, so a path drawn with a mouse behaves like a path drawn with a finger or with arrow keys. The matrix below summarizes the three input methods and their consequences.
| Input | Extend the path | Undo last step | Check current path | Restart the run |
|---|---|---|---|---|
| Pointer or mouse | Click an open neighbor, or press and drag across several open neighbors | Undo button | Check button or Enter | Restart button or R |
| Touch | Press and drag across adjacent open cells with a finger or stylus | Undo button | Check button or Enter | Restart button or R |
| Keyboard | Arrow keys extend one cell from the current endpoint | Delete or Backspace | Enter | R |
Under all three inputs the selection rules are atomic. Clicking the current cell, an already visited cell, a blocked cell, a nonadjacent cell, or a coordinate outside the board does nothing. Those invalid selections do not change the path and do not count as Check presses, which protects you from accidental scoring deductions. The on-screen touch targets are at least 44 pixels, so drag input stays usable on smaller touch screens.
Each chosen square displays its step number, the latest endpoint uses an accent treatment so the active end of the line is always visible, and a path-length counter shows how many open cells have been covered. Pressing X resets only the current level without affecting the points earned on earlier boards, while Delete or Backspace peels off the latest step without disturbing the fixed starting square.
The Two-Check Deadlock Rule and Mistake Recovery
Checking is deliberate in this fill the board puzzle, because a successful Check clears the level while an unsuccessful Check has a real cost that is easy to underestimate.
The system records the exact level-and-path signature of every incomplete Check. Rechecking the identical unchanged route does not add another error, so you can confirm a suspect path without penalty. A second different incomplete Check, however, deadlocks the run and requires a full Restart. That two-check rule exists to reward inspection over rapid guessing: it is faster to undo one step, study the board, and try a new line than to spam Check until the run locks.
Recovery tools before deadlock are simple and predictable. Undo removes the latest step without removing the fixed starting square, so you can rewind several moves in a row. Reset level (or the X key) returns the current board to its fixed starting square while keeping the points earned on earlier levels. Restart (or the R key) clears the entire run, returning to level one with zero points and no recorded checks.
After a level clears, the next level's fixed start is already in place and ready to extend. The boss key (double-tap Escape) swaps the board for a spreadsheet-style cover, which is useful when you need to step away without losing your place; pressing Escape twice again restores the game.
Practical Heuristics Before You Press Check
Because every move in a continuous-path fill is irreversible, the order of inspection matters as much as the order of moves. Three habits reliably reduce the number of failed checks and shorten the average solve.
Look at non-start corners first. A corner that is not the fixed start usually has only one open neighbor that matters, so leaving it must happen at the moment you have just entered it. If you visit a non-start corner and the only other open neighbor is already on the path, the corner is stranded and the only recovery is Undo or Reset level.
Count available neighbors before each move. Cells with very few remaining neighbors are the bottlenecks of the route. Treat each one like a junction: if you pass through it on the wrong step, the rest of the path can become disconnected. The path-length counter and step numbers help you see at a glance which cells still have unused neighbors.
Plan which side of a corridor closes first. Obstacles can create narrow passages with two open sides. Decide in advance which side will be completed before the corridor is crossed, because crossing first can seal the unvisited side away. Level 4 is explicitly designed around this idea.
When a move does strand a square, Undo returns one step at a time so you can watch the strand form and reverse it precisely. If multiple steps are tangled, Reset level returns to the fixed start without losing the points you already earned on earlier boards. The Fill One Line Puzzle game itself does not auto-solve, fill in the next square, or replace your line with an automatic route, so the inspection habit is the skill the variant actually trains.