A tower-defense map can let players shape enemy routes without running a fresh path search for every enemy. On Null Grid, a 32-by-18 map in Void Parry’s tower-defense mode, towers act as walls; a shared distance field points ground units toward the core while placement checks prevent the player from sealing off the entrance or trapping units already on the board.
Why a flow field fits a player-built maze
When many enemies share one destination, calculating a separate route for each one repeats work. A flow field instead stores, for each traversable cell, its distance to the goal. An enemy can then move toward a neighboring cell with a lower distance, taking movement cost into account. This shared-goal approach is a natural fit for tower defense, where player-placed towers keep changing the available routes. Red Blob Games explains the flow-field approach for tower-defense pathfinding; Nightdrive Labs describes its use in Null Grid.
As an Amazon Associate I earn from qualifying purchases.
Null Grid is described as a 32-by-18 open grid, with entry at the left and core at the right. That makes 576 cells. Towers occupy cells as walls, so the player designs the maze rather than following a fixed corridor. A longer route can give defenses more time to attack, but buying maze walls also leaves less money for damage upgrades. The source mentions cheaper barriers as a possible design tool, but does not provide prices or quantified balance results. Nightdrive Labs’ implementation article reports that “The whole feature is about 60 lines of pathfinding”; that describes its implementation, not the general effort required to build the complete feature.
How the 576-cell field is built
Start at the destination
The described implementation calculates distances outward from the exit using Dijkstra’s algorithm. It checks eight neighboring cells, assigning diagonal steps a cost of 1.41. This allows routes to use diagonals while recognizing that a diagonal move covers more distance than a straight step.
#1 Best Overall
- TENSE TEAMWORK – Work together to defend Castle Bravehold as monsters attack from all sides; trade cards, coordinate plans, and keep at least one tower standing to survive the siege as a united team.
- EDGE-OF-YOUR-SEAT EXPERIENCE – Face plagues, boulders, and boss monsters that disrupt your plans and force strategic choices; each turn adds suspense for both new and experienced players.
- CHOOSE YOUR CHALLENGE – Enjoy four modes: Co-op, Solo, Master Slayer competitive mode, and Overlord where one player controls the monsters; each option offers unique tension and replay value.
- TOWER DEFENSE THRILLS – Featuring 3D towers, vibrant monster tokens, and an illustrated board, this 45-minute adventure blends accessibility with satisfying strategic depth for 1–6 players.
- HEARTHSIDE HARMONY – Bring friends and family together with an easy-to-learn game that blends teamwork, excitement, and warm table energy; ideal for mixed-experience groups and repeat play.
Keep corner rules consistent
A diagonal step is disallowed if either of the two cells touched at the corner is blocked. The same restriction must apply both when constructing the field and when moving enemies. Otherwise, the field may direct a unit through a gap that movement rules should treat as blocked, making it look as if the unit clips through a tower corner. This is a rule in the Null Grid implementation, not a universal requirement: a game with different movement rules should use those same rules consistently in its path calculation and movement.
Rebuild when the maze changes
When a tower is built or sold, the described system rebuilds the field once. Ground enemies already on the grid choose new paths from their current cells; enemies that have not entered keep the shared route from the entry. Flying enemies ignore the field. This makes the field responsive to maze changes without treating each ground enemy’s route as a separately maintained search.
Rank #2
- STANDALONE TOWER-DEFENSE ADVENTURE: Jump into intense tower-defense action with Kingdom Rush: Elemental Uprising, a complete game for 1-4 players. No need for prior knowledge of Kingdom Rush: Rift in Time to enjoy this sequel's innovative challenges.
- COOPERATIVE STRATEGY FOR EVERYONE: Designed for both newcomers and seasoned gamers, featuring simple rules with deep tactical gameplay. Work together to defeat monster hordes through strategic planning and cooperation.
- VARIABLE DIFFICULTY & IRON CHALLENGES: Tailor your gameplay experience with multiple difficulty levels or test your skills with the Iron Challenges, demanding perfect strategies to conquer. Offers a tailored challenge for every type of player.
- UNIQUE HEROES & ASYMMETRICAL POWERS: Choose from unique, asymmetrical heroes, each with their own powers and playstyles. This diversity allows for dynamic gameplay and endless strategic combinations in every game session.
- EVOLVING CAMPAIGN & REPLAYABLE SCENARIOS: Engage in an evolving campaign with big boss fights, hero challenges, and more. Replayable scenarios ensure that every game is a new adventure, providing lasting entertainment and value.
How placement checks avoid trapping units
A placement can be legal for the player to click but still make the map impossible to navigate. The implementation checks a proposed tower before committing it:
- Temporarily block the candidate cell. Treat the proposed tower location as a wall for the test.
- Search outward from the exit. Run breadth-first search over the remaining traversable cells.
- Check the entry and current ground units. Reject the placement if the entry or any ground enemy already on the grid cannot reach the exit.
- Reject occupied cells. The system also refuses a placement directly on an enemy.
- Return a reason. A rejection can be explained to the player rather than silently failing.
Checking only the entry would miss a different failure: an enemy can already be on the far side of the proposed wall and become stranded even though new enemies still have a route. Conversely, checking current enemies without the entry could allow a placement that traps later waves. The described validation accounts for both.
Rank #3
- BUILD 3D SPIRES, DEFEND — Stack floor tiles into true vertical structures as you and up to three teammates race the clock to build four elemental spires, gather resources, and defend the Aethercore from waves of Aether Fiends. This cooperative tower building and defense game brings a real, freestanding 3D structure to your table, not just a flat board.
- PREMIUM WOOD & PUNCHBOARD — Every session includes 50 custom wooden pillar cubes and 41 wooden invader tokens, plus premium punchboard components: 38 floor tiles in three sizes and 4 3D spire toppers, along with a full set of character, boss, and adaptation cards. Eight custom acrylic 8-sided dice round out the set, adding a unique tactile touch to every roll.
- COOP PLAY, 1-4, 45 MIN — Play solo or team up with 2, 3, or 4 players in a fast-paced cooperative game built for teens and adults. Whether you're browsing board games for 2 or planning a full 4-player group, Aetherspire's 45-minute round makes it an easy addition to game night.
- STRATEGY GAME, AGES 14+ — Aetherspire is designed for players 14 and older, with the tactical depth and elemental storyline strategy gamers look for and rules that reward resource management and combination play over luck. Four distinct difficulty modes let your group start simple and grow into deeper strategy from session to session. A full rulebook is included to walk your group through setup.
- COMPLETE SET, NO EXTRAS — In addition to the premium wood and punchboard components listed above, the box includes the game board, active player token, 15 character cards, 16 boss cards, 6 spire adaptation cards, 4 player aid cards, 4 wall tokens, 5 disc tokens, 4 spire completion tokens, a cloth storage bag, and the full rulebook. Everything your group needs to start building, defending, and replaying Elementis is included from the first play.
Make the proposed route visible
Null Grid also describes a route preview while the player hovers over an empty cell. The candidate blockage is tested and a dashed yellow route is drawn. The preview is recalculated when the hovered cell or maze changes, rather than continuously for an unchanged proposal. This lets players see the path consequence before spending funds or committing to a wall.
Nightdrive Labs says it tests whether the preview route matches the route after placement. That is a reported test, not an independently reproduced result. Even so, equality between preview and post-placement routing is the key behavior to verify: both should use the same blocked-cell and diagonal-corner rules.
Rank #4
- EASY TO SET UP, EASY TO LEARN: Fire Tower takes seconds to set up and minutes to learn. The game is highly accessible because of its intuitive rules and visual instructions that show how each card works.
- UNIQUE THEME AND GAMEPLAY: Fire Tower brings players into a unique world of competitive firefighting. Use real firefighting techniques to defend your tower from the growing flames and take control of nature to send the fire towards your opponents to claim victory. Each turn, you must spread the fire in the direction of the wind and then play one of your action cards. Extinguish the blaze with fire engines, dispatch smoke jumpers, and build firebreaks. Harness the destructive force of nature to spread the flames away from your tower and towards your opponents.
- STRATEGIC AND FAST-PACED with HIGH REPLAYABILITY: Fire Tower takes 15-30 minutes to play. Every turn is a tense decision, as players must adjust to changing conditions in the forest and the plans of their opponents. Downtime is minimal, and players stay in the game even after their towers are destroyed, continuing to play as the vengeful Shadow of the Wood with special powers and a new path to victory. Try playing free-for-all or battle in teams of two to change up the strategy of the game.
- VIBRANT ARTWORK AND COMPONENTS: The game features original watercolors by fine artist Kevin Ruelle. The vivid colors perfectly match the beautiful, unique components and create a highly tactile experience that fully immerses players in the theme of the game. The game’s deluxe components include 135 glimmering fire gems, a printed cloth bag, custom meeples, an engraved wind die, and more.
- THE PERFECT GIFT: This game is a great gift for gamers, teens, adults and kids. It’s easy to introduce to new players, but has enough strategic depth to engage gamers as well, making it the perfect fit for family game night, Thanksgiving, or your next weekly get-together.
Choosing an approach for a different game
A shared field is most useful when many units move through a changing map toward the same goal. Separate path searches can be easier to think about as individual route requests, but they repeat work for a common destination. The cited sources do not provide a benchmark comparing those approaches on Null Grid, so the choice should be based on workload and movement rules rather than an assumed speed advantage.
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →The Null Grid article reports its plain O(n²) Dijkstra implementation runs “in well under a millisecond” on 576 cells and is run only when a tower is built or sold. That is the author’s result for that implementation, not an independent benchmark or a runtime guarantee. A larger grid may call for a priority queue backed by a heap; the article itself says a larger grid would need a real heap.
Best Value
- IMMERSIVE STRATEGY GAMEPLAY: Wandering Towers delivers a thoughtful strategy board game experience built around planning, timing, and tactical movement. Players must anticipate opponents’ actions, manage positioning, and make smart decisions each turn, creating constant engagement, meaningful choices, and a dynamic tabletop experience that rewards clever play and adaptability across repeated sessions.
- MAGICAL FANTASY THEME: Set in a whimsical fantasy world filled with moving towers and roaming wizards, this board game blends strategic depth with a lighthearted magical theme. The visual design and theme enhance immersion without overwhelming complexity, making it appealing to both experienced strategy fans and families seeking an imaginative and engaging game night option.
- HIGH INTERACTION AND REPLAY VALUE: Every session of Wandering Towers feels different due to player interaction and shifting board conditions. Strategic movement and competitive positioning ensure no two games play the same, encouraging experimentation with new tactics and approaches while maintaining balance, tension, and replayability that keeps players returning for more.
- HIGH INTERACTION AND REPLAY VALUE: Every session of Wandering Towers feels different due to player interaction and shifting board conditions. Strategic movement and competitive positioning ensure no two games play the same, encouraging experimentation with new tactics and approaches while maintaining balance, tension, and replayability that keeps players returning for more.
- COMPACT PLAYTIME WITH MEANINGFUL CHOICES: With a streamlined structure and focused objectives, Wandering Towers delivers a full strategic experience in a short play session. The efficient pacing keeps turns engaging and avoids downtime, making it easy to fit into busy schedules while still offering satisfying gameplay and thoughtful tactical decisions from start to finish.
For Apple-platform projects, Apple’s GameplayKit pathfinding guide presents a graph-based alternative: model the maze as a grid graph and remove wall nodes so paths route around them. Its GKGridGraph documentation covers the grid graph API. This is a platform-specific option, not a performance comparison with a custom flow field.
Quick Recap
What to preserve when implementing it
- Use a shared field when many ground units have the same destination.
- Recompute after a map change that alters traversable cells.
- Validate reachability for both the entry and ground units already on the map.
- Apply one consistent definition of legal diagonal movement in the field, placement test, preview, and actual movement.
- Show the likely route before a placement so players can understand the maze and its tradeoff between extra firing time and spending on damage.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
