How to make a co-op tower defense game in the browser

Net Forge, the open-source multiplayer browser game engine built for AI coding agents, is published on npm under @netforge with source at github.com/netnetdev/net-forge.

Run npm create netforge@latest my-td -- --template tower-defense --yes and you have a co-op tower defense: creeps walk a road in waves, up to four players build towers beside it with their own gold, every kill pays its builder a bounty and the rest of the team a share, and the team shares the lives. The road, the towers, the creeps and the waves are declarations in src/game.ts.

npm create netforge@latest my-td -- --template tower-defense --yes
cd my-td
npm run play

--template tower and --template td are accepted as the same template.

#What you get

#1. Start it

npm create netforge@latest my-td -- --template tower-defense --yes
cd my-td
npm run play -- --players 2

Click a tile to build the tower picked in the bar (keys 1 to 4), click a tower to upgrade or sell it, and press Space to call the next wave. On touch, tap a tile to aim and tap again to build.

#2. Draw a new road

import { towerBoard } from "@netforge/engine";

towerBoard({ size: [20, 12], path: [[0, 5], [6, 5], [6, 1], [13, 1], [13, 10], [19, 10]], lives: 20, gold: 100 });

Each step of the path changes x or y, never both, and the road starts and ends on the board's edge. Longer roads with more turns give towers more time; a straight road is hard.

#3. Add a tower and a creep

import { prefab, tower } from "@netforge/engine";

tower("sky_harpoon", { model: "style.lowpoly.rocket", kind: "bolt", cost: 120, damage: 60, rate: 0.4, range: 6 });
tower("bolt_post", {
  model: "style.lowpoly.turret",
  kind: "bolt",
  cost: 40,
  damage: 8,
  rate: 1.5,
  range: 3,
  levels: [{ cost: 45, damage: 14 }, { cost: 80, damage: 24, range: 3.5 }],
});
prefab("ironclad", { model: "style.lowpoly.robot_walker", health: 160, speed: 0.9, value: 14, damage: 3 });

Then bring the creep into the waves with a group such as { prefab: "ironclad", from: 8, count: 2, grow: 0.5, gap: 2 }. splash towers take a radius, frost towers a slow and slowSeconds, chain towers a number of chains.

#4. Test it

npm test

The playtest seats four builders and requires that they kill creeps, clear wave 3 and fire an earn rule within four minutes without losing the spire before wave 4. A road that is too short or a wave that is too strong fails here with the check that broke.

#How it works

Tower defense runs as a rules genre: a deterministic module the room server steps at 30 Hz, with integer state and a seeded random word. Your cursor is predicted so pointing never waits; builds, upgrades, sells and the ready vote are actions the server validates against your gold, the tile and who owns the tower. Creeps are networked entities the client interpolates. The same netcode, playtest and store serve every template. See Make a turn-based multiplayer game for rules genres in general and the tower defense template page for every file.

#Alternatives

• Phaser is a free, open-source 2D framework with WebGL and Canvas rendering and a large set of examples. Pick it for a single-player 2D tower defense with sprite art where you write the pathing and targeting yourself.
• Godot has a scene editor and navigation tools and exports to the web as WebAssembly with WebGL 2. Pick it when you want to design levels in an editor.

Pick Net Forge when you want the co-op version, with a server that owns the gold and the waves, already working.

#FAQ

#Can several people build on the same board?

Yes, up to four. Each builds with their own gold; kills pay the tower's builder a bounty and the rest of the team a share.

#How do I make the game harder?

Raise the waves' health growth, counts and grow, or lower starting gold and lives. A shorter prep time and breather make it faster.

#Do towers target the closest creep?

They target the creep furthest along the road within range, which is the usual tower defense rule and keeps leaks rare.

#Can I add a boss wave?

Yes. Declare a prefab with boss: true and add a group such as { prefab: "boilerback", from: 10, every: 10, count: 1, delay: 8 } for a boss every tenth wave.

#Can creeps take a path the players draw?

The road is declared in towerBoard({ path }). Players build beside it, not on it.