diff --git a/src/arena.c b/src/arena.c
index d64b8cdd..b90b6ab9 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -119,16 +119,16 @@ void arena_decommit_unused_blocks(struct arena *arena)
 
 void arena_set_readonly(struct arena *arena)
 {
-    sys_memory_set_committed_readonly(arena->base, arena->committed);
 #if RTC
     arena->readonly = true;
 #endif
+    sys_memory_set_committed_readonly(arena->base, arena->committed);
 }
 
 void arena_set_readwrite(struct arena *arena)
 {
+    sys_memory_set_committed_readwrite(arena->base, arena->committed);
 #if RTC
     arena->readonly = false;
 #endif
-    sys_memory_set_committed_readwrite(arena->base, arena->committed);
 }
diff --git a/src/game.c b/src/game.c
index b017d7e6..f5d94a52 100644
--- a/src/game.c
+++ b/src/game.c
@@ -15,6 +15,8 @@ GLOBAL struct {
     struct atomic_i32 game_thread_shutdown;
     struct sys_thread game_thread;
 
+    /* Double buffered game state */
+    struct world prev_world;  /* Last tick */
     struct world world;
 
     /* Game thread input */
@@ -51,6 +53,7 @@ struct game_startup_receipt game_startup(struct mixer_startup_receipt *mixer_sr,
 
     /* Initialize tick transmission */
     world_alloc(&G.published_tick);
+    arena_set_readonly(&G.published_tick.entity_store->arena);
     G.published_tick_mutex = sys_mutex_alloc();
 
     G.world.timescale = GAME_TIMESCALE;
@@ -132,7 +135,7 @@ INTERNAL void spawn_test_entities(void)
 
     /* Child 1 */
     {
-        struct v2 pos = V2(0.25, -0.25);
+        struct v2 pos = V2(1, 0);
         struct v2 size = V2(1, 1);
         f32 r = PI / 4;
 
@@ -172,7 +175,11 @@ INTERNAL void publish_game_tick(void)
 {
     __prof;
     struct sys_lock lock = sys_mutex_lock_e(&G.published_tick_mutex);
-    world_copy_replace(&G.published_tick, &G.world);
+    arena_set_readwrite(&G.published_tick.entity_store->arena);
+    {
+        world_copy_replace(&G.published_tick, &G.world);
+    }
+    arena_set_readonly(&G.published_tick.entity_store->arena);
     atomic_u64_eval_exchange(&G.published_tick_id, G.published_tick.tick_id);
     sys_mutex_unlock(&lock);
 }
diff --git a/src/sys.h b/src/sys.h
index 2840a843..6bbf1777 100644
--- a/src/sys.h
+++ b/src/sys.h
@@ -167,7 +167,10 @@ void *sys_memory_commit(void *address, u64 size);
  * address space) */
 void sys_memory_decommit(void *address, u64 size);
 
+/* Mark committed pages as readonly */
 void sys_memory_set_committed_readonly(void *address, u64 size);
+
+/* Mark committed pages as readable & writeable (default for committed memory) */
 void sys_memory_set_committed_readwrite(void *address, u64 size);
 
 /* ========================== *