diff --git a/game/event.go b/game/event.go
new file mode 100644
index 0000000..529d2c1
--- /dev/null
+++ b/game/event.go
@@ -0,0 +1,138 @@
+package game
+
+import (
+	"math/rand"
+
+	"github.com/tolelom/catacombs/dungeon"
+	"github.com/tolelom/catacombs/entity"
+)
+
+func (s *GameSession) EnterRoom(roomIdx int) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.state.Floor.CurrentRoom = roomIdx
+	dungeon.UpdateVisibility(s.state.Floor)
+	room := s.state.Floor.Rooms[roomIdx]
+
+	if room.Cleared {
+		return
+	}
+
+	switch room.Type {
+	case dungeon.RoomCombat:
+		s.spawnMonsters()
+		s.state.Phase = PhaseCombat
+		s.state.CombatTurn = 0
+	case dungeon.RoomBoss:
+		s.spawnBoss()
+		s.state.Phase = PhaseCombat
+		s.state.CombatTurn = 0
+	case dungeon.RoomShop:
+		s.generateShopItems()
+		s.state.Phase = PhaseShop
+	case dungeon.RoomTreasure:
+		s.grantTreasure()
+		room.Cleared = true
+	case dungeon.RoomEvent:
+		s.triggerEvent()
+		room.Cleared = true
+	case dungeon.RoomEmpty:
+		room.Cleared = true
+	}
+}
+
+func (s *GameSession) spawnMonsters() {
+	count := 1 + rand.Intn(5) // 1~5 monsters
+	floor := s.state.FloorNum
+	s.state.Monsters = make([]*entity.Monster, count)
+
+	// Pick appropriate monster type for floor
+	var mt entity.MonsterType
+	switch {
+	case floor <= 5:
+		mt = entity.MonsterSlime
+	case floor <= 10:
+		mt = entity.MonsterSkeleton
+	case floor <= 14:
+		mt = entity.MonsterOrc
+	default:
+		mt = entity.MonsterDarkKnight
+	}
+
+	// Solo mode: 50% HP
+	for i := 0; i < count; i++ {
+		m := entity.NewMonster(mt, floor)
+		if s.state.SoloMode {
+			m.HP = m.HP / 2
+			if m.HP < 1 {
+				m.HP = 1
+			}
+			m.MaxHP = m.HP
+		}
+		s.state.Monsters[i] = m
+	}
+}
+
+func (s *GameSession) spawnBoss() {
+	var mt entity.MonsterType
+	switch s.state.FloorNum {
+	case 5:
+		mt = entity.MonsterBoss5
+	case 10:
+		mt = entity.MonsterBoss10
+	case 15:
+		mt = entity.MonsterBoss15
+	case 20:
+		mt = entity.MonsterBoss20
+	default:
+		mt = entity.MonsterBoss5
+	}
+	boss := entity.NewMonster(mt, s.state.FloorNum)
+	if s.state.SoloMode {
+		boss.HP = boss.HP / 2
+		boss.MaxHP = boss.HP
+	}
+	s.state.Monsters = []*entity.Monster{boss}
+}
+
+func (s *GameSession) grantTreasure() {
+	// Random item for each player
+	for _, p := range s.state.Players {
+		if rand.Float64() < 0.5 {
+			p.Inventory = append(p.Inventory, entity.Item{
+				Name: "Iron Sword", Type: entity.ItemWeapon, Bonus: 3 + rand.Intn(6),
+			})
+		} else {
+			p.Inventory = append(p.Inventory, entity.Item{
+				Name: "Iron Shield", Type: entity.ItemArmor, Bonus: 2 + rand.Intn(4),
+			})
+		}
+	}
+}
+
+func (s *GameSession) generateShopItems() {
+	s.state.ShopItems = []entity.Item{
+		{Name: "HP Potion", Type: entity.ItemConsumable, Bonus: 30, Price: 20},
+		{Name: "Iron Sword", Type: entity.ItemWeapon, Bonus: 3 + rand.Intn(6), Price: 40 + rand.Intn(41)},
+		{Name: "Iron Shield", Type: entity.ItemArmor, Bonus: 2 + rand.Intn(4), Price: 30 + rand.Intn(31)},
+	}
+}
+
+func (s *GameSession) triggerEvent() {
+	// Random event: 50% trap, 50% blessing
+	for _, p := range s.state.Players {
+		if p.IsDead() {
+			continue
+		}
+		if rand.Float64() < 0.5 {
+			// Trap: 10~20 damage
+			dmg := 10 + rand.Intn(11)
+			p.TakeDamage(dmg)
+		} else {
+			// Blessing: heal 15~25
+			heal := 15 + rand.Intn(11)
+			p.Heal(heal)
+		}
+	}
+}
diff --git a/game/lobby.go b/game/lobby.go
new file mode 100644
index 0000000..af03b81
--- /dev/null
+++ b/game/lobby.go
@@ -0,0 +1,94 @@
+package game
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+)
+
+type RoomStatus int
+
+const (
+	RoomWaiting RoomStatus = iota
+	RoomPlaying
+)
+
+type LobbyRoom struct {
+	Code    string
+	Name    string
+	Players []string
+	Status  RoomStatus
+	Session *GameSession
+}
+
+type Lobby struct {
+	mu    sync.RWMutex
+	rooms map[string]*LobbyRoom
+}
+
+func NewLobby() *Lobby {
+	return &Lobby{rooms: make(map[string]*LobbyRoom)}
+}
+
+func (l *Lobby) CreateRoom(name string) string {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	code := generateCode()
+	for l.rooms[code] != nil {
+		code = generateCode()
+	}
+	l.rooms[code] = &LobbyRoom{
+		Code:   code,
+		Name:   name,
+		Status: RoomWaiting,
+	}
+	return code
+}
+
+func (l *Lobby) JoinRoom(code, playerName string) error {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	room, ok := l.rooms[code]
+	if !ok {
+		return fmt.Errorf("room %s not found", code)
+	}
+	if len(room.Players) >= 4 {
+		return fmt.Errorf("room %s is full", code)
+	}
+	if room.Status != RoomWaiting {
+		return fmt.Errorf("room %s already in progress", code)
+	}
+	room.Players = append(room.Players, playerName)
+	return nil
+}
+
+func (l *Lobby) GetRoom(code string) *LobbyRoom {
+	l.mu.RLock()
+	defer l.mu.RUnlock()
+	return l.rooms[code]
+}
+
+func (l *Lobby) ListRooms() []*LobbyRoom {
+	l.mu.RLock()
+	defer l.mu.RUnlock()
+	result := make([]*LobbyRoom, 0, len(l.rooms))
+	for _, r := range l.rooms {
+		result = append(result, r)
+	}
+	return result
+}
+
+func (l *Lobby) RemoveRoom(code string) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	delete(l.rooms, code)
+}
+
+func generateCode() string {
+	const letters = "ABCDEFGHJKLMNPQRSTUVWXYZ"
+	b := make([]byte, 4)
+	for i := range b {
+		b[i] = letters[rand.Intn(len(letters))]
+	}
+	return string(b)
+}
diff --git a/game/lobby_test.go b/game/lobby_test.go
new file mode 100644
index 0000000..25b6b7d
--- /dev/null
+++ b/game/lobby_test.go
@@ -0,0 +1,40 @@
+package game
+
+import "testing"
+
+func TestCreateRoom(t *testing.T) {
+	lobby := NewLobby()
+	code := lobby.CreateRoom("Test Room")
+	if len(code) != 4 {
+		t.Errorf("Room code length: got %d, want 4", len(code))
+	}
+	rooms := lobby.ListRooms()
+	if len(rooms) != 1 {
+		t.Errorf("Room count: got %d, want 1", len(rooms))
+	}
+}
+
+func TestJoinRoom(t *testing.T) {
+	lobby := NewLobby()
+	code := lobby.CreateRoom("Test Room")
+	err := lobby.JoinRoom(code, "player1")
+	if err != nil {
+		t.Errorf("Join failed: %v", err)
+	}
+	room := lobby.GetRoom(code)
+	if len(room.Players) != 1 {
+		t.Errorf("Player count: got %d, want 1", len(room.Players))
+	}
+}
+
+func TestJoinRoomFull(t *testing.T) {
+	lobby := NewLobby()
+	code := lobby.CreateRoom("Test Room")
+	for i := 0; i < 4; i++ {
+		lobby.JoinRoom(code, "player")
+	}
+	err := lobby.JoinRoom(code, "player5")
+	if err == nil {
+		t.Error("Should reject 5th player")
+	}
+}
diff --git a/game/session.go b/game/session.go
new file mode 100644
index 0000000..ee6712c
--- /dev/null
+++ b/game/session.go
@@ -0,0 +1,133 @@
+package game
+
+import (
+	"sync"
+
+	"github.com/tolelom/catacombs/dungeon"
+	"github.com/tolelom/catacombs/entity"
+)
+
+type GamePhase int
+
+const (
+	PhaseExploring GamePhase = iota
+	PhaseCombat
+	PhaseShop
+	PhaseResult
+)
+
+type PlayerAction struct {
+	Type      ActionType
+	TargetIdx int
+}
+
+type ActionType int
+
+const (
+	ActionAttack ActionType = iota
+	ActionSkill
+	ActionItem
+	ActionFlee
+	ActionWait
+)
+
+type GameState struct {
+	Floor      *dungeon.Floor
+	Players    []*entity.Player
+	Monsters   []*entity.Monster
+	Phase      GamePhase
+	FloorNum   int
+	TurnNum    int
+	CombatTurn int // reset per combat encounter
+	SoloMode   bool
+	GameOver   bool
+	Victory    bool
+	ShopItems  []entity.Item
+}
+
+type GameSession struct {
+	mu       sync.Mutex
+	state    GameState
+	actions  map[string]PlayerAction // playerName -> action
+	actionCh chan playerActionMsg
+}
+
+type playerActionMsg struct {
+	PlayerName string
+	Action     PlayerAction
+}
+
+func NewGameSession() *GameSession {
+	return &GameSession{
+		state: GameState{
+			FloorNum: 1,
+		},
+		actions:  make(map[string]PlayerAction),
+		actionCh: make(chan playerActionMsg, 4),
+	}
+}
+
+// StartGame determines solo mode from actual player count at game start
+func (s *GameSession) StartGame() {
+	s.mu.Lock()
+	s.state.SoloMode = len(s.state.Players) == 1
+	s.mu.Unlock()
+	s.StartFloor()
+}
+
+func (s *GameSession) AddPlayer(p *entity.Player) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.state.Players = append(s.state.Players, p)
+}
+
+func (s *GameSession) StartFloor() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.state.Floor = dungeon.GenerateFloor(s.state.FloorNum)
+	s.state.Phase = PhaseExploring
+	s.state.TurnNum = 0
+
+	// Revive dead players at 30% HP
+	for _, p := range s.state.Players {
+		if p.IsDead() {
+			p.Revive(0.30)
+		}
+	}
+}
+
+func (s *GameSession) GetState() GameState {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.state
+}
+
+func (s *GameSession) SubmitAction(playerName string, action PlayerAction) {
+	s.actionCh <- playerActionMsg{PlayerName: playerName, Action: action}
+}
+
+// BuyItem handles shop purchases
+func (s *GameSession) BuyItem(playerName string, itemIdx int) bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.state.Phase != PhaseShop || itemIdx < 0 || itemIdx >= len(s.state.ShopItems) {
+		return false
+	}
+	item := s.state.ShopItems[itemIdx]
+	for _, p := range s.state.Players {
+		if p.Name == playerName && p.Gold >= item.Price {
+			p.Gold -= item.Price
+			p.Inventory = append(p.Inventory, item)
+			return true
+		}
+	}
+	return false
+}
+
+// LeaveShop exits the shop phase
+func (s *GameSession) LeaveShop() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.state.Phase = PhaseExploring
+	s.state.Floor.Rooms[s.state.Floor.CurrentRoom].Cleared = true
+}
diff --git a/game/session_test.go b/game/session_test.go
new file mode 100644
index 0000000..0a4630b
--- /dev/null
+++ b/game/session_test.go
@@ -0,0 +1,29 @@
+package game
+
+import (
+	"testing"
+	"time"
+
+	"github.com/tolelom/catacombs/entity"
+)
+
+func TestSessionTurnTimeout(t *testing.T) {
+	s := NewGameSession()
+	p := entity.NewPlayer("test", entity.ClassWarrior)
+	s.AddPlayer(p)
+	s.StartFloor()
+
+	// Don't submit any action, wait for timeout
+	done := make(chan struct{})
+	go func() {
+		s.RunTurn()
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		// Turn completed via timeout
+	case <-time.After(7 * time.Second):
+		t.Error("Turn did not timeout within 7 seconds")
+	}
+}
diff --git a/game/turn.go b/game/turn.go
new file mode 100644
index 0000000..b4c0973
--- /dev/null
+++ b/game/turn.go
@@ -0,0 +1,271 @@
+package game
+
+import (
+	"math/rand"
+	"time"
+
+	"github.com/tolelom/catacombs/combat"
+	"github.com/tolelom/catacombs/dungeon"
+	"github.com/tolelom/catacombs/entity"
+)
+
+const TurnTimeout = 5 * time.Second
+
+func (s *GameSession) RunTurn() {
+	s.mu.Lock()
+	s.state.TurnNum++
+	s.state.CombatTurn++
+	s.actions = make(map[string]PlayerAction)
+	aliveCount := 0
+	for _, p := range s.state.Players {
+		if !p.IsDead() {
+			aliveCount++
+		}
+	}
+	s.mu.Unlock()
+
+	// Collect actions with timeout
+	timer := time.NewTimer(TurnTimeout)
+	collected := 0
+	for collected < aliveCount {
+		select {
+		case msg := <-s.actionCh:
+			s.mu.Lock()
+			s.actions[msg.PlayerName] = msg.Action
+			s.mu.Unlock()
+			collected++
+		case <-timer.C:
+			goto resolve
+		}
+	}
+	timer.Stop()
+
+resolve:
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	// Default action for players who didn't submit: Wait
+	for _, p := range s.state.Players {
+		if !p.IsDead() {
+			if _, ok := s.actions[p.Name]; !ok {
+				s.actions[p.Name] = PlayerAction{Type: ActionWait}
+			}
+		}
+	}
+
+	s.resolvePlayerActions()
+	s.resolveMonsterActions()
+}
+
+func (s *GameSession) resolvePlayerActions() {
+	var intents []combat.AttackIntent
+
+	// Track which monsters were alive before this turn (for gold awards)
+	aliveBeforeTurn := make(map[int]bool)
+	for i, m := range s.state.Monsters {
+		if !m.IsDead() {
+			aliveBeforeTurn[i] = true
+		}
+	}
+
+	// Check if ALL alive players chose flee — only then the party flees
+	fleeCount := 0
+	aliveCount := 0
+	for _, p := range s.state.Players {
+		if p.IsDead() {
+			continue
+		}
+		aliveCount++
+		if action, ok := s.actions[p.Name]; ok && action.Type == ActionFlee {
+			fleeCount++
+		}
+	}
+	if fleeCount == aliveCount && aliveCount > 0 {
+		if combat.AttemptFlee() {
+			s.state.Phase = PhaseExploring
+			return
+		}
+		// Flee failed — all fleeing players waste their turn, continue to monster phase
+		return
+	}
+
+	for _, p := range s.state.Players {
+		if p.IsDead() {
+			continue
+		}
+		action, ok := s.actions[p.Name]
+		if !ok {
+			continue
+		}
+
+		switch action.Type {
+		case ActionAttack:
+			intents = append(intents, combat.AttackIntent{
+				PlayerATK:  p.EffectiveATK(),
+				TargetIdx:  action.TargetIdx,
+				Multiplier: 1.0,
+				IsAoE:      false,
+			})
+		case ActionSkill:
+			switch p.Class {
+			case entity.ClassWarrior:
+				// Taunt: mark all monsters to target this warrior
+				for _, m := range s.state.Monsters {
+					if !m.IsDead() {
+						m.TauntTarget = true
+						m.TauntTurns = 2
+					}
+				}
+			case entity.ClassMage:
+				intents = append(intents, combat.AttackIntent{
+					PlayerATK:  p.EffectiveATK(),
+					TargetIdx:  -1,
+					Multiplier: 0.8,
+					IsAoE:      true,
+				})
+			case entity.ClassHealer:
+				if action.TargetIdx >= 0 && action.TargetIdx < len(s.state.Players) {
+					s.state.Players[action.TargetIdx].Heal(30)
+				}
+			case entity.ClassRogue:
+				// Scout: reveal neighboring rooms
+				currentRoom := s.state.Floor.Rooms[s.state.Floor.CurrentRoom]
+				for _, neighborIdx := range currentRoom.Neighbors {
+					s.state.Floor.Rooms[neighborIdx].Visited = true
+				}
+			}
+		case ActionItem:
+			// Use first consumable from inventory
+			for i, item := range p.Inventory {
+				if item.Type == entity.ItemConsumable {
+					p.Heal(item.Bonus)
+					p.Inventory = append(p.Inventory[:i], p.Inventory[i+1:]...)
+					break
+				}
+			}
+		case ActionFlee:
+			// Individual flee does nothing if not unanimous (already handled above)
+		case ActionWait:
+			// Defensive stance — no action
+		}
+	}
+
+	if len(intents) > 0 && len(s.state.Monsters) > 0 {
+		combat.ResolveAttacks(intents, s.state.Monsters)
+	}
+
+	// Award gold only for monsters that JUST died this turn
+	for i, m := range s.state.Monsters {
+		if m.IsDead() && aliveBeforeTurn[i] {
+			goldReward := 5 + s.state.FloorNum
+			if goldReward > 15 {
+				goldReward = 15
+			}
+			for _, p := range s.state.Players {
+				if !p.IsDead() {
+					p.Gold += goldReward
+				}
+			}
+			// Boss kill: drop relic
+			if m.IsBoss {
+				s.grantBossRelic()
+			}
+		}
+	}
+
+	// Filter out dead monsters
+	alive := make([]*entity.Monster, 0)
+	for _, m := range s.state.Monsters {
+		if !m.IsDead() {
+			alive = append(alive, m)
+		}
+	}
+	s.state.Monsters = alive
+
+	// Check if combat is over
+	if len(s.state.Monsters) == 0 {
+		s.state.Floor.Rooms[s.state.Floor.CurrentRoom].Cleared = true
+		// Check if this was the boss room -> advance floor
+		if s.state.Floor.Rooms[s.state.Floor.CurrentRoom].Type == dungeon.RoomBoss {
+			s.advanceFloor()
+		} else {
+			s.state.Phase = PhaseExploring
+		}
+	}
+}
+
+func (s *GameSession) advanceFloor() {
+	if s.state.FloorNum >= 20 {
+		s.state.Phase = PhaseResult
+		s.state.Victory = true
+		s.state.GameOver = true
+		return
+	}
+	s.state.FloorNum++
+	s.state.Floor = dungeon.GenerateFloor(s.state.FloorNum)
+	s.state.Phase = PhaseExploring
+	s.state.CombatTurn = 0
+	// Revive dead players at 30% HP
+	for _, p := range s.state.Players {
+		if p.IsDead() {
+			p.Revive(0.30)
+		}
+	}
+}
+
+func (s *GameSession) grantBossRelic() {
+	relics := []entity.Relic{
+		{Name: "Vampiric Ring", Effect: entity.RelicHealOnKill, Value: 5, Price: 100},
+		{Name: "Power Amulet", Effect: entity.RelicATKBoost, Value: 3, Price: 120},
+		{Name: "Iron Ward", Effect: entity.RelicDEFBoost, Value: 2, Price: 100},
+		{Name: "Gold Charm", Effect: entity.RelicGoldBoost, Value: 5, Price: 150},
+	}
+	for _, p := range s.state.Players {
+		if !p.IsDead() {
+			r := relics[rand.Intn(len(relics))]
+			p.Relics = append(p.Relics, r)
+		}
+	}
+}
+
+func (s *GameSession) resolveMonsterActions() {
+	if s.state.Phase != PhaseCombat {
+		return
+	}
+	for _, m := range s.state.Monsters {
+		if m.IsDead() {
+			continue
+		}
+		targetIdx, isAoE := combat.MonsterAI(m, s.state.Players, s.state.CombatTurn)
+		if isAoE {
+			// Boss AoE: 0.5x damage to all
+			for _, p := range s.state.Players {
+				if !p.IsDead() {
+					dmg := combat.CalcDamage(m.ATK, p.EffectiveDEF(), 0.5)
+					p.TakeDamage(dmg)
+				}
+			}
+		} else {
+			if targetIdx >= 0 && targetIdx < len(s.state.Players) {
+				p := s.state.Players[targetIdx]
+				if !p.IsDead() {
+					dmg := combat.CalcDamage(m.ATK, p.EffectiveDEF(), 1.0)
+					p.TakeDamage(dmg)
+				}
+			}
+		}
+		m.TickTaunt()
+	}
+
+	// Check party wipe
+	allPlayersDead := true
+	for _, p := range s.state.Players {
+		if !p.IsDead() {
+			allPlayersDead = false
+			break
+		}
+	}
+	if allPlayersDead {
+		s.state.Phase = PhaseResult
+	}
+}