A few things

Make child entites work.

z-sort entity list before rendering.

entites have a creation timestamp.

include/Engine/Entity/Renderable.h

@@ -8,9 +8,15 @@ namespace Engine {
 }
 
 class Engine::Renderable : public Entity {
+protected:
+    // Higher numbers are farther to the back.
+    unsigned int depth = 0;
+public:
+    [[nodiscard]] unsigned int GetDepth() const { return depth; }
+    void SetDepth(unsigned int new_depth) { depth = new_depth; }
 public:
     // *must* be overridden.
     virtual void Render() = 0;
 public:
-    explicit Renderable(const Vector2& position, float rotation = 0.0f) : Entity(position, rotation) {}
+    explicit Renderable(const Vector2& position, unsigned int depth = 0, float rotation = 0.0f) : Entity(position, rotation), depth(depth) {}
 };