path: root/research/forts-gameplay.org

#+TITLE: Forts — Gameplay Mechanics Research
#+AUTHOR: Forts Clone Project
#+DATE: 2025-06-19
#+OPTIONS: toc:3 num:t

* Overview

Forts is a 2D physics-based real-time strategy game developed by EarthWork Games
(Australia), released April 19, 2017 on Steam. Over 1 million copies sold, 91-92%
positive reviews (22,000+). Described as "Worms meets bridge-building meets RTS."

Core tagline: ~Build, research, and blast your opponent's fort to rubble.~

* Core Game Loop

1. Build resource structures (mines on metal deposits, wind turbines for energy)
2. Construct your fortress from physics-simulated materials
3. Research the tech tree to unlock weapons and devices
4. Fortify defenses and position weapons
5. Fire weapons manually at the enemy's weak points
6. Destroy the enemy's Reactor (core) to win

The loop is real-time, not turn-based. All construction happens under fire — you
build and fight simultaneously. The single-pointer multitasking model means
you're constantly switching between building, repairing, aiming, and managing
resources.

* Win Conditions

- Primary: Destroy the opponent's Reactor (core)
- Methods to destroy the reactor:
  - Direct damage (lasers, cannons, missiles that hit the reactor)
  - Indirect damage (fire spread, splash damage that reaches the reactor)
  - Structural collapse (destroying supports so the reactor falls — falling
    damage destroys it)
- Some DLC/custom missions add capture-point objectives

* Resources & Economy

**Resources**

| Resource    | How to Obtain                                            | Used For                          |
|-------------+----------------------------------------------------------+-----------------------------------|
| Metal       | Mines placed on metal deposits scattered across the map  | Building blocks, weapons, devices |
| Energy      | Wind turbines (higher placement = more energy)           | Powering weapons, shields, devices|
| Oil         | Campaign narrative focus, not a direct gameplay resource | Story context                     |
| Rare Metal   | Moonshot DLC resource for reactor upgrades              | Advanced tech                     |

**Resource Mechanics**
- Metal is a continuous trickle — more mines = more metal/sec
- Energy scales with turbine height AND lack of obstruction (build tall, build
  free of blockage for max efficiency)
- Energy is stored in Battery devices
- Metal is stored in Storage devices
- All construction costs both metal and energy
- Repairing damaged structures costs resources proportional to damage

* Building & Construction

**Core Building Philosophy**
- Free-form, real-time construction — not grid-snapped, not pre-placed
- Every plank, beam, rope, and panel is a separate physics body
- Structures must be anchored to something stable or they collapse
- You can build literally any shape and size
- Construction is a skill — good builders create stable, defensible forts;
  bad builders create structures that collapse under their own weight or from
  a single well-placed shot

**Known Building Materials**

Categorized by function:

| Category        | Materials                                    | Properties                         |
|-----------------+----------------------------------------------+------------------------------------|
| Structural      | Wooden beams, Metal beams, Planks             | Basic building blocks, have HP     |
| Reinforcement   | Metal plates, Armor panels                    | Higher HP, resist damage types     |
| Tension         | Ropes/Cables                                  | Connect points, provide stability  |
| Defense         | Sandbags, Doors (openable/closable)           | Absorb/redirect damage             |
| Resource        | Batteries, Metal Storage                      | Store energy/metal buffers         |
| Functional      | Wind Turbines, Mines, Tech Buildings          | Generate/process resources         |

**Rope/Cable Mechanics (from Chinese wiki)**
- Cost: 5 metal + 25 energy per cell
- HP: 50 per segment
- Vulnerable to: fire, cutting weapons
- Primary use: suspending cannons, reactors, and heavy structures
- Provides tension stability — a rope under tension keeps structures upright
- If a rope is destroyed, whatever it was supporting may collapse
- Can catch fire and burn through

**Structural Integrity**
- Every piece has independent physics — not a unified "health bar" for the fort
- Destroying key support pieces causes cascading collapse
- A well-placed shot on a load-bearing beam can bring down an entire weapon
  platform, reactor, or section of the fort
- This is THE core strategic mechanic — you're not just depleting HP, you're
  finding and exploiting structural weaknesses

**Building Actions**
- Build: Place a material piece, costs resources instantly
- Repair: Fix damaged piece, cost proportional to damage amount
- Recycle/Delete: Remove piece, recover partial resources

**Physics Properties of Buildings**
- Gravity applies to everything
- Pieces have mass, friction, collision shapes
- Weight distribution matters — top-heavy forts tip over
- Pieces connected via joints (distance joints, revolute/weld equivalents)
- Destruction of joints triggers cascading failure

* Combat Systems

**Weapons — 16+ base weapons across base game and DLCs**

| Weapon            | Type        | Characteristics                                    |
|-------------------+-------------+----------------------------------------------------|
| Machine Gun       | Hitscan     | High fire rate, low building damage, can shoot down|
|                   |             | incoming mortars/missiles (point-defense)          |
| Sniper Rifle      | Hitscan     | Precision, high single-point damage, used to guide |
|                   |             | Swarm Missiles to target                           |
| Cannon            | Projectile  | Heavy damage, rips through armor layers            |
| Mortar            | Projectile  | High building damage, slow projectile, arcing shot |
| Swarm Missiles    | Guided      | Self-aiming cluster, accurate, tracks targets      |
| Plasma Laser      | Beam        | Cuts through entire bases in a line, countered by  |
|                   |             | energy shields                                     |
| Flak              | Point-Def   | Anti-projectile, shoots down incoming shells       |
| EMP Missile       | Special     | Disables devices/weapons temporarily               |
| Minigun           | Hitscan     | Extreme fire rate, suppression                     |
| Howitzer          | Projectile  | Moonshot DLC — heavy artillery                     |
| Smoke Bombs       | Utility     | Moonshot DLC — obscures vision                     |
| Magnabeam         | Beam        | Moonshot DLC — magnetic manipulation               |
| Buzzsaw           | Melee/Phys  | Moonshot DLC — physical cutting                    |
| Dome              | Shield      | High Seas DLC — protective energy dome             |
| Orbital Laser     | Beam        | High Seas DLC — orbital strike                     |
| Deckguns          | Projectile  | High Seas DLC — naval cannons                      |
| Naval Missiles    | Projectile  | High Seas DLC — ship-launched missiles             |

**Damage Types**
- Kinetic (bullets, shells): splashes, penetration
- Energy (lasers, plasma): cuts linearly, blocked by shields
- Fire: spreads, burns ropes and wood, continuous damage over time
- Explosive: area damage, knocks pieces loose, stress damage
- EMP: disables without physical destruction
- Physical/Falling: collision damage from collapsing structures

**Aiming Model**
- All weapons are manually aimed by the player
- No auto-targeting (except Swarm Missiles and point-defense)
- Aiming is skill-based — angle, power, and timing matter
- The sniper uses a 2D scope view
- Weapons have limited firing arcs based on mounting position

**Defensive Systems**
- Armor panels: absorb kinetic damage
- Energy shields: block lasers and plasma
- Flak: shoot down incoming projectiles
- Sandbags: cheap damage absorption
- Machine guns: can be set to point-defense mode
- Structural redundancy: building with multiple load paths so one broken
  connection doesn't collapse everything

* Commanders & Factions

**12+ Commanders (base game + DLCs)**

Each commander has unique active abilities and passive traits:

| Commander      | Theme / Playstyle              |
|----------------+--------------------------------|
| Warthog        | Aggressive firepower           |
| Armourdillo    | Defensive resilience           |
| Shockenaugh    | Cunning/tactical               |
| Architect      | Building mastery               |
| (8+ more)      | Various hybrid styles          |

Commanders differentiate playstyles — they're like fighting game characters.
One player's optimal fort design may be completely different from another's
based on commander choice.

**Campaign Factions (narrative only)**
- Eagle Empire (fictional USA)
- Dragon Army (fictional China)
- Iron Bear Alliance (fictional Russia)
- Black Penguin Oil Company (antagonist — private military)

In multiplayer, factions are cosmetic/narrative — the mechanical differentiation
comes from commander choice.

* Tech Tree & Progression

- Starts with basic weapons and materials
- Research unlocks progressively more powerful weapons, devices, and materials
- Two strategic paths:
  1. Rush tech — invest resources in research, get powerful weapons fast but
     with a weaker fort
  2. Turtle — invest in fortification, outlast the enemy, tech up slowly
- Tech buildings must be physically built on your fort and can be destroyed
- Weapons Factory: unlocks weapon production
- Upgrade Center: improves existing weapons (damage, fire rate, etc.)
- Tech prerequisites create build-order strategy (like StarCraft tech tree)
- Research costs both metal and energy

* Game Modes

| Mode             | Players      | Description                                   |
|------------------+--------------+-----------------------------------------------|
| Campaign         | 1            | 28 missions + tutorial, 3 factions, story     |
| Skirmish         | 1 vs AI      | 3 AI difficulty levels                        |
| Multiplayer TDM  | Up to 8 (2 teams) | Each player has own fort & resources      |
| Multiplayer Co-op| Up to 8 (2 teams) | Team shares base and resources            |
| Ranked 1v1       | 2            | Bi-monthly seasons, leaderboard, medals       |
| Sandbox          | 1            | Free build, no enemies, experiment mode       |

- ~70 official maps included
- Hundreds of community-made maps via Steam Workshop
- Official tournaments (XXII+ held as of 2022) with prize pools
- Community tournaments including AI vs AI tournaments

* Maps & Environment

**Map Features**
- 2D side-view maps
- Terrain with metal deposit nodes scattered throughout
- Destructible terrain in some maps
- Background environments with dynamic elements (17+ dynamic backgrounds in
  High Seas DLC)
- Water/physics in High Seas DLC — floating naval forts with buoyancy

**Starting Conditions**
- Each player starts with a Reactor (core) at a fixed position
- Initial small fort structure around the reactor
- Nearby metal deposits for initial mining
- Symmetric or asymmetric map layouts depending on map design

* Physics & Simulation

**Structural Physics (the game's signature feature)**
- Every building piece is an independent rigid body
- Pieces connected via joints (distance joints for ropes, weld joints for
  attached planks, revolute joints for pivoting structures)
- Joint breaking triggers cascading destruction — this is the core of
  the gameplay
- Stress and strain accumulate on connections
- Weight distribution matters — unbalanced loading causes tilt and collapse
- Counterweights can stabilize structures (hanging heavy objects off the
  opposite side)

**Projectile Physics**
- Real ballistic trajectories (not hitscan except for machine gun/sniper)
- Gravity affects all projectiles
- Splash damage has radial falloff
- Impact force can knock pieces loose from joints
- Penetration mechanics — some projectiles punch through multiple layers

**Environmental Physics**
- Water buoyancy (High Seas DLC) — structures float, ships have hulls
- Fire propagation — spreads along connected flammable materials
- Wind affects turbine efficiency

**Determinism**
- The game uses a deterministic physics simulation for multiplayer
- Lockstep netcode (inferred — common for physics-based RTS games of this era)
- Same initial state + same inputs = identical simulation on all peers

* Multiplayer & Netcode

**Multiplayer Structure**
- Up to 8 players per lobby
- Two teams (any split: 1v1, 2v2, 3v3, 4v4, 4v2, etc.)
- Team Deathmatch: separate forts
- Team Co-op: shared base and resource pool
- Ranked 1v1 with bi-monthly season resets
- Leaderboard ranks and cosmetic medal rewards

**Inferred Netcode Architecture**
- Likely deterministic lockstep: all clients run full simulation, only inputs
  are transmitted
- This is standard for physics-heavy RTS games (Age of Empires, Spring RTS,
  Supreme Commander all use this approach)
- Bandwidth scales with player count and input rate, NOT entity count — can
  support massive numbers of physics bodies with tiny packets
- Downside: one slow connection freezes the game for everyone

**Observed Multiplayer Issues**
- Community reports mention desyncs and lag in some conditions
- No ranked matchmaking — new players can match against veterans
- Bugs and balance issues noted in multiplayer reviews

* Modding System

Forts has a deliberate, first-class modding architecture. This is one of the
most important sections for designing a clone — Forts' moddability is a major
reason for its longevity.

**Modding Architecture**

- Layered Lua loading system:
  1. Base game Lua files load first
  2. Each active mod's equivalent files load on top
  3. Mods can override or extend variables and tables from earlier files

- Priority system (1-10, default 5):
  - Lower numbers load first
  - Higher numbers load last, have "final say"
  - Same priority: alphabetical order

**Mod File Structure**
#+begin_src text
my_mod/
├── mod.lua                  # Metadata (name, category, priority, selectable)
├── db/
│   └── constants.lua        # Physics constants (gravity, drag, max angles)
├── devices/
│   ├── device_list.lua      # Device definitions, costs, prerequisites
│   └── my_device.lua        # Individual device config
├── weapons/
│   ├── weapons_list.lua     # Weapon definitions, fire rates, projectiles
│   └── projectile_list.lua  # Projectile damage, splash, multipliers
├── materials/
│   └── building_materials.lua # Material properties, HP, costs, build times
└── ui/textures/
    └── HUD/                 # Custom HUD icons and sprites
#+end_src

**Key Lua API Functions**
| Function                       | Purpose                                    |
|--------------------------------+--------------------------------------------|
| =table.insert()=                | Add new items to game tables               |
| =IndexOfWeapon()/IndexOfDevice()=| Find existing item position for insertion  |
| =FindProjectile()=              | Look up projectile to modify               |
| =RegisterApplyMod()=            | Schedule function to run after all mods load |
| =DeregisterApplyMod()=          | Remove previously registered ApplyMod      |
| =ButtonSprite()/DetailSprite()= | Create HUD sprites from textures           |
| =InterpolateTable()=            | Interpolate time-keyed data (beam weapons) |

**What Can Be Modded**
- ✅ New devices (weapons, buildings, tools)
- ✅ New weapons with custom projectiles
- ✅ New building materials with custom properties
- ✅ New projectiles with custom damage/splash/physics
- ✅ Beam weapons with custom thickness/damage functions
- ✅ Game balance (costs, damage, build times, production rates)
- ✅ Physics constants (gravity, drag, max angles)
- ✅ Visuals (sprites, textures, HUD elements)
- ✅ Game rules, prerequisites, tech tree requirements
- ✅ Maps (hundreds of community maps)
- ✅ AI behavior (AI tournaments exist)
- ✅ Training missions

**What's NOT Currently Moddable**
- ❌ Commanders & Factions (special mod type, not Workshop-supported)
- ❌ Custom campaigns (not yet available)
- ❌ Environments/backgrounds
- ❌ Localization/languages

**Modding Tools**
- Built-in Map Editor
- VS Code extension: "Forts API extension" (autocomplete, intellisense, docs)
- Community tools: sprite pivot tools, HUD icon generators, mod starter scripts,
  structure duplication tools (GitHub: SamsterBirdies/forts-modding-tools)

**Design Lesson for Our Clone**
The layered Lua loading with priority system is elegant. It allows:
- Mods to extend (add to tables) or override (replace values)
- Multiple mods to coexist (priority + alphabetical ordering)
- Modders to reference other mods' content (RegisterApplyMod runs after all
  mods load, letting mods modify content added by later-loading mods)

* DLC Content Summary

| DLC              | Price   | Key Additions                                     |
|------------------+---------+---------------------------------------------------|
| Moonshot         | $7.99   | 4 weapons, 3 commanders, portals, new gamemode    |
| High Seas        | $9.99   | Naval forts, buoyancy physics, 6 weapons, campaign|
| Pro HUD          | $2.99   | UI reskins, new sounds                            |
| Tons of Guns     | Free    | Large weapon pack                                 |
| Repair Station   | Free    | Repair robot pet device                           |

* Key Design Takeaways for Our Clone

1. **Physics is THE feature.** The structural integrity system is what makes
   Forts special. Building isn't cosmetic — it IS the gameplay. Every support
   beam matters. Every rope under tension is a potential weakness.

2. **Modding from day one.** Forts shipped with Lua modding and Steam Workshop
   integration. Our clone should design modding into the core architecture,
   not bolt it on later.

3. **Manual aiming creates skill expression.** RTS + manual aim is unusual and
   creates a unique skill ceiling. Players who can build well AND aim well
   dominate.

4. **Resource nodes drive map strategy.** Metal deposits on maps force players
   to expand and defend territory — prevents turtling being the only strategy.

5. **Commanders create matchup variety.** 12+ commanders with unique abilities
   means 144+ matchup permutations in 1v1. This is enormous for replayability.

6. **The single-pointer model is a double-edged sword.** Players must multitask
   building, repairing, aiming, and resource management with one cursor. Our
   UI needs to make this feel fluid, not frustrating.

7. **Reactor-as-win-condition creates tense moments.** The reactor is fragile
   and central. Every battle becomes a desperate defense of your own reactor
   while trying to expose theirs.

8. **2D side-view simplifies the problem space.** You only need to worry about
   structural physics in 2 dimensions, which makes determinism and netcode
   much more tractable than a 3D equivalent.