Stop Players from Blocking the Path in a Godot Tower Defense

The softlock is one tower away

Most maze-style tower-defense bugs do not arrive as a crash. The player places one more tower, the last gap closes, and the next wave has nowhere legal to go. Maybe the enemies freeze. Maybe they walk through a wall because some fallback path still says "close enough". Either way, the game just let the player break the contract.

The fix is not to trust the placement and hope pathfinding recovers later. The fix is to validate the tower before it becomes part of the game state. Treat the new tower as a candidate blocker, ask whether the route still exists, and only commit the tower if the answer is yes.

This article uses a small Godot 4.7 web demo for that rule. You can place towers, watch the path reroute, try to seal the route, and see the validator reject the move with a reason instead of leaving the wave to discover the problem for you.

The rule

In a Godot tower defense, accept a tower placement only if every required spawn-to-goal route still exists after temporarily applying that tower as a blocker.

That one sentence is the whole contract. The player can bend the route, lengthen it, and create a maze. The player cannot seal every valid route from a spawn to the exit.

The implementation is deliberately boring: mark the candidate cell solid, ask the pathfinder whether the required route still exists, then either commit the tower or restore the cell. The important detail is timing. Validation happens before the tower becomes trusted game state, not after enemies discover the route is impossible.

Temporary blockers are the safe shape

A tower placement is a transaction. The game proposes a change to the navigation grid, validates the changed world, and only then commits the tower. If validation fails, the grid is restored and the player gets a clear rejection.

That sounds obvious, but many tower-defense prototypes do the reverse: spawn the tower first, rebuild or repath later, and hope the next wave exposes the problem. The failure mode is messy because navigation, economy, visuals, and enemy state have already accepted a placement that should never have existed.

Treat the pathfinder as the rule checker. The renderer can show the ghost tower. The economy can reserve the price. The real blocker should not be committed until the route contract passes.

func can_place_tower(cell: Vector2i) -> bool:
    if not _can_block_cell(cell):
        return false

    astar_grid.set_point_solid(cell, true)
    var ok := _all_required_routes_exist()
    astar_grid.set_point_solid(cell, false)

    return ok

Commit only after validation

The production version usually combines validation and commit in one small function so a failed placement always restores the exact previous state. That matters if you have reserved cells, editor previews, build phases, undo, or multiplayer authority around placement.

The example below is intentionally small. It assumes the surrounding project owns checks such as buildable terrain, tower overlap, cost, and placement cooldown. The path rule should be one gate in that pipeline, not the only gate.

func try_commit_tower(cell: Vector2i) -> bool:
    if not _can_block_cell(cell):
        return false

    var was_solid := astar_grid.is_point_solid(cell)
    astar_grid.set_point_solid(cell, true)

    if not _all_required_routes_exist():
        astar_grid.set_point_solid(cell, was_solid)
        return false

    towers[cell] = true
    return true

AStarGrid2D state boundary

For a tile tower defense, AStarGrid2D is a good primitive because a placed tower is exactly a cell-level blocker. Use set_point_solid(cell, true) for the speculative block and get_id_path() or get_point_path() for the route check.

Do not rebuild the grid for a tower toggle. In Godot's stable AStarGrid2D.update() contract, changes to grid parameters such as region, cell_size, offset, and cell_shape require update(). Point data such as set_point_solid() is the runtime occupancy layer. That is the boundary that keeps a placement check cheap.

If your game uses NavigationServer2D regions instead of a grid, the same rule still applies conceptually, but the transaction is heavier. You need a navigation representation where the temporary blocker is visible to the query you use for validation.

Multiple spawns and exits

Most real tower-defense maps have more than one spawn, more than one exit, or wave-specific rules. Do not hard-code a single route if the design rule is broader than that.

Define the rule explicitly. For each spawn, decide whether it needs a route to every goal or at least one acceptable goal. Then validate that contract under the temporary blocker. The code should make the design policy visible.

var route_rules: Array[Dictionary] = [
    {
        "spawn": Vector2i(0, 6),
        "goals": [Vector2i(19, 6)],
    },
    {
        "spawn": Vector2i(0, 10),
        "goals": [Vector2i(19, 6), Vector2i(19, 10)],
    },
]

func _all_required_routes_exist() -> bool:
    for rule in route_rules:
        var spawn: Vector2i = rule["spawn"]
        var goals: Array = rule["goals"]
        var at_least_one_goal := false

        for goal in goals:
            if not astar_grid.get_id_path(spawn, goal).is_empty():
                at_least_one_goal = true
                break

        if not at_least_one_goal:
            return false

    return true

Full validation vs incremental validation

The simplest validator runs every required route query after every candidate placement. That is full validation. It is easy to trust and often good enough for a small playable grid.

Incremental validation is the optimization: keep the current accepted route and first ask whether the candidate tower touches anything that could invalidate it. If the candidate cell is not on the current route and does not affect the local route corridor, the placement can often be accepted without a full search. If the local check is uncertain or the candidate may seal the maze, fall back to a full confirmation.

The right production policy is not "never run full validation". It is "run full validation only when the cheap evidence is not enough." The fallback is what keeps the incremental approach correct.

What the benchmark says

The demo includes a narrow benchmark because the claim here is not just ergonomic. On the small 20x14 playable board, the incremental path usually scanned one cell, while full validation scanned the whole 280-cell board. On the 128x128 stress board, full validation scanned 16,384 cells on every sampled placement.

The dense caveat matters. At 128x128 and 75% target fill, incremental validation still had a median of one scanned cell, but its p95 rose to 16,402 cells because some placements needed full-confirm territory. That is the expected tail: incremental validation is a fast path with a correctness fallback, not a promise that the worst case disappears.

The full-validation elapsed time also drifts down as fill rises, from 17.035 ms median at 0% fill to 10.210 ms at 75% fill on the 128x128 board. That does not mean full validation became less work in the route-rule sense. Fewer reachable cells remain for the path search to expand, while the validator still scans the 16,384-cell board; cells scanned is the portable proof, and elapsed ms is this demo's custom A* timing.

What the demo proves and what it does not

The browser demo is the visual proof: place towers, watch the path reroute, and try to seal the route. The benchmark JSON is the measurement source. The downloadable Godot project is the audit trail.

This is not a broad claim about Godot's internal AStarGrid2D throughput. The benchmark uses the demo's custom validation harness and a custom A* with a linear-scan open list so it can count scanned cells and compare validation policies directly. Use the relative shape and the cells-scanned metric as the takeaway.

The practical conclusion is conservative: start with full validation for correctness, add incremental validation when placement checks become common enough to matter, and keep full validation as the fallback for dense or ambiguous cases.