Fill One Line Puzzle is a browser-based route-planning game that asks players to draw one continuous orthogonal path through every open cell across five fixed grids, scoring exactly 1,000 points when all five levels are cleared. Each puzzle starts at a numbered square and your job is to extend a single line through every open square exactly once, never crossing a blocked cell, never revisiting a square, and never jumping diagonally. The five levels are original fixed boards with different obstacle arrangements, and each one is independently verified by a depth-first search so you can be certain a solution exists from the fixed starting cell. The game supports pointer dragging, touch input, and full keyboard play, so the same rules work whether you draw with a mouse on a desktop or tap with a thumb on a phone. This guide explains what the puzzle actually is, how each input type controls the line, the exact scoring rule that produces 1,000 points across the five levels, and the planning habit that keeps a run alive from level one through the final board.
What You Are Drawing on Each Grid
The puzzle asks for one continuous path, not a network of separate lines, and that single distinction drives every decision you make on the board. Each level is a small rectangular grid with a fixed starting cell, a set of dark blocked squares that act as walls, and a set of open squares you must visit exactly once. The path begins at the numbered start and grows one cell at a time by moving into an orthogonally adjacent open cell, meaning the next square must share an edge with the current endpoint and lie directly above, below, left, or right. Diagonal jumps are not allowed, and blocked squares can never be entered no matter which side you approach them from.
This is a cell-coverage puzzle rather than an edge-tracing puzzle, and the difference is visible in the numbered labels that appear on every square you claim. In an Euler-style one-line drawing, a vertex can be visited more than once as long as each required edge is used correctly. In Fill One Line Puzzle, the open cells are the things being covered, and each open cell may appear only once in the route. The numbered labels show the order in which your path visited the squares, which makes it easy to see at a glance whether you have accidentally reused a cell or skipped one.
The five boards are designed to feel different without changing the rule. Level one is an open three-by-three introduction with no interior walls. Later boards introduce a missing cell, protected corners, narrower corridors, and longer routes so the planning you need to do increases gradually. Every level uses a distinct obstacle arrangement and a fixed starting cell so each attempt is reproducible, and an independent depth-first search oracle enumerates every legal orthogonal path from each start to confirm that a complete route exists.
How to Play Fill One Line Puzzle
Open the board and find the numbered start before drawing anything, then extend the path one cell at a time using the input method you prefer. The numbered step labels appear on every square you claim so you can confirm visually that your line is staying on the grid. If you need to backtrack, several tools help without ending the run, and you can check the route as soon as the last open cell is covered.
Open Fill One Line Puzzle and follow these steps for each level.
- Identify the numbered starting cell on the current board; every route begins there.
- Move into an orthogonally adjacent open cell using a click, a drag, a tap, or an arrow key.
- Continue extending the path one cell at a time without crossing a blocked square or revisiting an open cell.
- Use Undo, Delete, or Backspace to remove the latest step if you trap yourself.
- Cover every open cell, then choose Check path or press Enter to verify.
- Clear five levels to earn exactly 1,000 points, and press R only when you want to restart the entire run.
Rules: Cells, Walls, and the No-Revisit Limit
Every interaction in Fill One Line Puzzle comes from a small set of explicit rules, and the rules apply identically to pointer, touch, and keyboard input. The path starts at a fixed numbered cell and may only grow by adding an open cell that is orthogonally adjacent to the current endpoint. Blocked cells are walls: they cannot be entered from any direction, and they block movement on both sides of their edge. An already used cell is treated as if it were a wall for the remainder of the level, so revisiting a cell is impossible even when the path leaves and returns near it.
A few selection actions are deliberately ignored because they cannot change the path. Clicking the current endpoint, an already visited cell, a blocked cell, a nonadjacent cell, or a coordinate outside the board does nothing. Those invalid selections do not alter the path, do not count as checks, and do not advance any error counter. This means you can experiment freely with clicks without worrying that a misclick will ruin a level.
Checking is a deliberate action that closes the route. A level clears only when the path contains every open square, and clearing a level awards exactly 200 points and immediately loads the fixed start of the next level. Checking an incomplete route does not clear the level, but it records the route's signature. Rechecking the identical unchanged route does not add another error because the duplicate signature is deduplicated. A second different incomplete check deadlocks the run, so the game rewards inspection before checking rather than rapid guessing.
Controls for Pointer, Touch, and Keyboard Play
The same rules accept three different input methods so the puzzle works on a desktop, a tablet, and a phone. Pointer input uses click and drag: a single click on an adjacent open cell extends the path by one square, while pressing and holding the pointer and dragging across the grid adds each square as the pointer enters it, provided that square is open, unused, and next to the current endpoint. Touch input follows the same local interaction model, so a finger drag adds cells under the same adjacency rule. Buttons in the interface are sized to at least 44 pixels to satisfy common touch guidelines, and they call native browser controls so the interaction feels the same across devices.
Keyboard play mirrors the grid rules one key at a time. The arrow keys attempt to extend the route by one cell from its current endpoint in the direction you press. An arrow key that would leave the board, enter a blocked square, or revisit a used square has no effect and the path does not change. Delete and Backspace remove the latest step without removing the fixed start, so you can back up one cell at a time. Press X to reset only the current level, press Enter to check the current path, and press R to restart the complete five-level run.
The two input families share the same actions, and the table below shows how each action is performed with a pointer or touch device and how it is performed with a keyboard.
| Action | Pointer or Touch | Keyboard |
|---|---|---|
| Extend by one cell | Click an adjacent open cell | Arrow keys |
| Extend several cells | Press, hold, and drag across the grid | Not applicable |
| Undo the last step | Click Undo | Delete or Backspace |
| Reset the current level | Click Reset | Press X |
| Check the current path | Click Check path | Press Enter |
| Restart the full run | Click Restart | Press R |
| Swap to a cover screen | Click the boss-key control | Press Escape twice |
Pointer dragging is the fastest way to draw a long straight section, but arrow keys let you step the route one cell at a time and undo mistakes with a single keystroke.
Scoring: 200 Per Level and 1,000 for the Run
Score in Fill One Line Puzzle is exact and fully disclosed. Clearing a single level awards exactly 200 points and immediately loads the fixed start of the next board. There are five levels in the run, so clearing all five produces exactly 1,000 points. The points from earlier cleared levels persist across the rest of the run, so you do not lose progress when you advance to a harder board. Pressing X to reset only the current level does not erase the points earned on earlier levels.
The score does not decrease for incorrect checks, but the run itself can lock. The first distinct incomplete check records the current level-and-path signature, and the second different incomplete check deadlocks the run and requires a full Restart. A Restart clears the run to level one, resets the score to zero, clears the recorded checks, and reloads the original starting path. Because the deadlocked run cannot be saved by Reset level, the two-check rule encourages you to verify the route visually before pressing Check rather than guessing and correcting.
Completion and deadlock are terminal states. Once a level is cleared or a run is locked, movement, selection, undo, reset-level, and check commands no longer alter the game. A Restart creates a fresh run with level one, zero points, no recorded checks, and the original starting path.
Strategy for Tracing One Path Through Five Grids
The strategy that works on every board is the same: look at the unvisited corners and the cells with very few available neighbors before extending the path. A non-start corner often needs to be approached at the right moment because leaving it later can be impossible once the path closes around it. Obstacles can create bottlenecks, so it helps to decide which side of a narrow passage should be completed first and work outward from there. This is also why the first move matters: starting in the wrong direction can isolate a cell on the far side of a wall.
A second habit is to treat Undo as a normal tool rather than a confession of failure. Delete, Backspace, and the Undo button all remove the latest step without removing the fixed start, and using them costs no points. Reset level does not erase the points from earlier levels either, so it is safe to abandon a partial route on a hard board and try again. If you are about to press Check and the path still leaves a corner unreachable, undo a few steps and reroute through that corner first. The first distinct incomplete check does not lock the run, so this kind of mid-route inspection is part of normal play.
The five fixed boards are designed to teach this habit gradually. Level one is an open three-by-three introduction where the planning load is light, and later boards add a missing cell, protected corners, narrower corridors, and longer routes so the planning you need to do increases step by step. By the final level, the same corner-first habit is what separates a clean 1,000-point run from a deadlocked restart.
Because every level has a different layout, the following table summarises how the official boards differ in focus.
| Level | What the board adds | Planning focus |
|---|---|---|
| 1 | Open 3x3 introduction | Learn the adjacency rule |
| 2 | A missing cell | Pick which side of the gap to fill first |
| 3 | Protected corners | Approach each corner at the right moment |
| 4 | Narrower corridors | Choose which side of the bottleneck first |
| 5 | Longer route | Keep the corner-first habit across the full board |
Mistakes That End a Run Early
Most failed runs come from a few predictable errors. The most common is starting the path in the wrong direction and trapping a cell on the far side of a wall before the path can reach it. Another is treating the puzzle like an edge-tracing puzzle and trying to pass through a corner twice; because every open cell may appear only once, that move simply dead-ends the route. A third is rushing to press Check before every cell is covered, which records an incomplete signature and leaves only one safe recheck.
A subtler mistake is using the Undo button or Delete key expecting it to clear the board entirely. Undo removes only the latest step, and the fixed starting cell cannot be removed. If the path is in real trouble, the right tool is Reset level or the X key, both of which reload the fixed starting cell without erasing earlier level points. The boss key, triggered by pressing Escape twice, replaces the board with a spreadsheet-style cover and pressing Escape twice again returns to the game, but it does not affect the run itself.
Finally, the game runs entirely in the browser and does not upload a board, request an account, call an external API, or use a paid service. The best completed score is stored locally when the browser allows it, so progress survives a refresh on the same device but does not sync elsewhere. Knowing this keeps the focus where it belongs: on the order of the moves, not on the chrome around them.