diff --git a/include/td/simulation/ServerSimulation.h b/include/td/simulation/ServerSimulation.h
index 2114fd1..aaadbad 100644
--- a/include/td/simulation/ServerSimulation.h
+++ b/include/td/simulation/ServerSimulation.h
@@ -1,21 +1,34 @@
 #pragma once
 
+#include <mutex>
 #include <td/game/World.h>
 
 namespace td {
 namespace sim {
 
 // TODO: OnEnd signal
-class ServerSimulation {
+/**
+ * \note This class is thread safe
+ */
+class ServerSimulation : public protocol::CommandHandler {
   private:
+	std::mutex m_Mutex;
 	game::World& m_World;
 	std::uint64_t m_StepTime;
 	std::uint64_t m_CurrentTime;
+	std::vector<td::protocol::LockStep> m_History;
+
+	using protocol::CommandHandler::Handle;
 
   public:
 	ServerSimulation(game::World& a_World, std::uint64_t a_StepTime);
 
-	void Update();
+	protocol::packets::LockStepsPacket Update();
+
+	// no checks are done !
+
+	virtual void Handle(const protocol::commands::SpawnTroopCommand& a_SpawnTroop) override;
+	virtual void Handle(const protocol::commands::PlaceTowerCommand& a_PlaceTower) override;
 };
 
 }  // namespace sim
diff --git a/src/td/simulation/ServerSimulation.cpp b/src/td/simulation/ServerSimulation.cpp
new file mode 100644
index 0000000..497d866
--- /dev/null
+++ b/src/td/simulation/ServerSimulation.cpp
@@ -0,0 +1,36 @@
+#include <td/simulation/ServerSimulation.h>
+
+namespace td {
+namespace sim {
+
+ServerSimulation::ServerSimulation(game::World& a_World, std::uint64_t a_StepTime) :
+	m_World(a_World), m_StepTime(a_StepTime), m_CurrentTime(0) {}
+
+protocol::packets::LockStepsPacket ServerSimulation::Update() {
+	std::lock_guard<std::mutex> lock(m_Mutex);
+	
+	m_World.Tick(m_History[m_CurrentTime], FpFloat(m_StepTime) / FpFloat(1000));
+	m_CurrentTime++;
+
+	std::array<protocol::LockStep, LOCKSTEP_BUFFER_SIZE> nextSteps;
+	std::copy(m_History.begin() + m_CurrentTime, m_History.begin() + m_CurrentTime + nextSteps.size(), nextSteps.begin());
+	return {m_CurrentTime, std::move(nextSteps)};
+}
+
+template<typename T>
+void AddToCommandHistory(protocol::LockStep& a_LockStep, const T& a_Cmd) {
+	a_LockStep.push_back({std::make_shared<T>(a_Cmd)});
+}
+
+void ServerSimulation::Handle(const protocol::commands::SpawnTroopCommand& a_SpawnTroop) {
+	std::lock_guard<std::mutex> lock(m_Mutex);
+	AddToCommandHistory(m_History[m_CurrentTime + LOCKSTEP_BUFFER_SIZE], a_SpawnTroop);
+}
+
+void ServerSimulation::Handle(const protocol::commands::PlaceTowerCommand& a_PlaceTower) {
+	std::lock_guard<std::mutex> lock(m_Mutex);
+	AddToCommandHistory(m_History[m_CurrentTime + LOCKSTEP_BUFFER_SIZE], a_PlaceTower);
+}
+
+}  // namespace sim
+}  // namespace td