Co-op Roguelike Deckbuilder — Design Spec

A simplified, web-based Slay the Spire-inspired deckbuilder with a co-op twist. 2-4 players fight through encounters together in real-time, using both solo and cooperative cards.

Core Concept

The gimmick: co-op card play. Players share combat encounters and play cards in real-time. The card system includes cooperative mechanics (Assist, Chain, Rally) that reward coordination without requiring it — simple to play, deep in interaction.

Target: Browser-based, playable among friends. Prototype scope.

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1. Core Game Loop

1. Lobby — Host creates a room (4-letter code), 1-3 others join. Host starts when ready.
2. Map — Branching path of ~8 nodes. Host picks the path. Node types:
   • Combat — standard enemy encounter (most common)
   • Elite — harder enemies, better rewards
   • Rest — heal HP
   • Boss — final encounter of the run
3. Combat:
   • Each player starts with 3 energy and draws 5 cards from their own deck.
   • Player phase — all players play cards freely in real-time. Cards resolve instantly when played. Players see each other's actions as they happen. Each player hits "end turn" when done.
   • Enemy phase — once all players end their turn, enemies execute their telegraphed intents, hitting all players.
   • Repeat until all enemies die or all players die.
4. Reward — each player picks 1 card from a choice of 3 to add to their deck.
5. Repeat until boss is defeated (win) or all players are dead (lose).

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2. Cards & Combat Mechanics

Resources (per player)

Shared Resource

Card Categories

Status Effects

Standard:

• Strength — +X damage on attacks
• Vulnerable — take 50% more damage
• Weak — deal 25% less damage
• Poison — lose X HP at start of turn, decreases by 1 each turn

Co-op:

• Linked — when one Linked player blocks, the other gains half that block

Starter Deck (10 cards per player)

Enemies

• All enemies show intent icons before acting (attack, buff, debuff).
• Enemies attack all players by default.
• Enemy HP scales with player count (x1 for 2p, x1.5 for 3p, x2 for 4p).
• V1 scope: ~5 basic enemy types, 2 elites, 1 boss.

Extensibility

Cards are defined as data (not hardcoded logic). Adding new cards, status effects, enemies, or categories requires adding data entries — not rewriting game code. Future additions: relics/items, shops, events, character classes.

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3. Multiplayer & Networking

Tech: Colyseus server (Node.js/TypeScript) + WebSocket connections.

Room Flow

1. Host creates a room, gets a 4-letter room code (e.g., "ABCD").
2. Friends join by entering the code.
3. Host sees a lobby with connected players, hits "Start" when ready.

State Sync

• Server owns all game state (HP, decks, draw/discard piles, enemies, map, Rally pool).
• Player plays a card: client sends { action: "playCard", cardId, targetId? }.
• Server validates (enough energy? card in hand? valid target?), resolves the effect, broadcasts updated state to all clients.
• All clients see card plays and results in real-time.

Turn Management

• Player phase is open — everyone plays cards freely and simultaneously.
• Each player sends "endTurn" when done.
• Server waits for all players to end turn, then runs enemy phase and broadcasts results.

Reconnection

• Colyseus handles reconnection natively.
• If a player disconnects mid-combat, their turn auto-ends after ~30s timeout.

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4. Tech Stack & Project Structure

Stack

Project Structure

/packages
  /shared        — card data, types, constants
  /server        — Colyseus server, game logic, room handlers
  /client        — Phaser game, scenes, UI components

Phaser Scenes

Hosting

• Server: Any Node.js host (Railway, Fly.io, or local)
• Client: Static hosting (Vercel, Netlify, or served by the same Node server)

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5. Art & Assets

Target aesthetic: Hand-drawn / sketch style, with pixel art fallback.

Asset Sources

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6. V1 Scope Summary

In scope:

• 1 shared character class
• ~30-40 cards (mix of solo + co-op)
• 10-card starter deck
• 3 energy / 5 card draw per turn
• ~5 basic enemies, 2 elites, 1 boss
• 8-node branching map (Combat, Elite, Rest, Boss)
• 2-4 player real-time co-op
• Lobby with room codes
• Card rewards after combat
• Rally shared resource
• Chain and Assist card mechanics

Out of scope (future):

• Shops, events, random encounters
• Relics / items / potions
• Multiple character classes
• Card upgrades
• Leaderboards / accounts
• Mobile support