diff --git a/source/Game.cpp b/source/Game.cpp
index 2dbd85b..f0b5d95 100644
--- a/source/Game.cpp
+++ b/source/Game.cpp
@@ -41,10 +41,10 @@ Game::Game(const Vector2i& resolution) :
 	mTileManager.generate();
 	for (int i = 0; i < 500; i += 50) {
 		mCollection.insert(std::shared_ptr<Sprite>(new Cover(Vector2f(i, i), Vector2i(20, 20),
-				mWorld)), Collection::LEVEL_STATIC);
+				mWorld)));
 	}
-	mCollection.insert(std::shared_ptr<Sprite>(new Enemy(mWorld, Vector2f(400.0f, 200.0f), mCollection)),
-			Collection::LEVEL_ACTOR);
+	mCollection.insert(std::shared_ptr<Sprite>(new Enemy(mWorld, Vector2f(400.0f, 200.0f),
+			mCollection)));
 }
 
 /**
diff --git a/source/abstract/Physical.cpp b/source/abstract/Physical.cpp
index 9dc65e1..87b1207 100755
--- a/source/abstract/Physical.cpp
+++ b/source/abstract/Physical.cpp
@@ -117,9 +117,9 @@ Physical::getDelete() const {
 /**
  * Returns the Physical::Category of this object.
  */
-uint16
+Physical::Category
 Physical::getCategory() const {
-	return mBody->GetFixtureList()->GetFilterData().categoryBits;
+	return (Category) mBody->GetFixtureList()->GetFilterData().categoryBits;
 }
 
 /**
diff --git a/source/abstract/Physical.h b/source/abstract/Physical.h
index dc35d1c..5acd851 100755
--- a/source/abstract/Physical.h
+++ b/source/abstract/Physical.h
@@ -49,14 +49,15 @@ public:
 
 	/**
 	 * Categories of physical objects, for Box2D collision filtering.
+	 * The order of categories is also used for render order.
 	 *
 	 * @warning An object may only have one category.
 	 */
 	enum Category {
-		CATEGORY_NONSOLID = 0,
 		CATEGORY_WORLD = 1 << 1,
-		CATEGORY_ACTOR = 1 << 2,
-		CATEGORY_PARTICLE = 1 << 3
+		CATEGORY_NONSOLID = 1 << 2,
+		CATEGORY_PARTICLE = 1 << 3,
+		CATEGORY_ACTOR = 1 << 4
 	};
 
 	/**
@@ -76,7 +77,7 @@ public:
 	Vector2f getSpeed() const;
 	float getAngle() const;
 	bool getDelete() const;
-	uint16 getCategory() const;
+	Category getCategory() const;
 
 	virtual bool doesCollide(Physical& other);
 	virtual void onCollide(Physical& other, uint16 category);
diff --git a/source/particle/Emitter.cpp b/source/particle/Emitter.cpp
index 03dc5be..5901f60 100755
--- a/source/particle/Emitter.cpp
+++ b/source/particle/Emitter.cpp
@@ -19,5 +19,5 @@ Emitter::~Emitter() {
  */
 void
 Emitter::emit() {
-	mCollection.insert(createParticle(), Collection::LEVEL_PARTICLE);
+	mCollection.insert(createParticle());
 }
diff --git a/source/sprite/Enemy.cpp b/source/sprite/Enemy.cpp
index dd81dc7..49daf48 100644
--- a/source/sprite/Enemy.cpp
+++ b/source/sprite/Enemy.cpp
@@ -20,8 +20,7 @@ Enemy::Enemy(b2World& world, const Vector2f& position, Collection& collection) :
 
 Enemy::~Enemy() {
 	// Insert here to avoid altering b2d data during timestep.
-	mCollection.insert(std::shared_ptr<Sprite>(new Body(mWorld, getPosition())),
-			Collection::LEVEL_STATIC);
+	mCollection.insert(std::shared_ptr<Sprite>(new Body(mWorld, getPosition())));
 }
 
 void
diff --git a/source/util/Collection.cpp b/source/util/Collection.cpp
index debb13d..762822d 100755
--- a/source/util/Collection.cpp
+++ b/source/util/Collection.cpp
@@ -14,10 +14,11 @@
  * An object can't be inserted more than once at the same level.
  */
 void
-Collection::insert(std::shared_ptr<Sprite> drawable, Level level) {
-	auto item = std::find(mDrawables[level].begin(), mDrawables[level].end(), drawable);
-	if (item == mDrawables[level].end()) {
-		mDrawables[level].push_back(drawable);
+Collection::insert(std::shared_ptr<Sprite> drawable) {
+	Physical::Category cat = drawable->getCategory();
+	auto item = std::find(mDrawables[cat].begin(), mDrawables[cat].end(), drawable);
+	if (item == mDrawables[cat].end()) {
+		mDrawables[cat].push_back(drawable);
 	}
 }
 
diff --git a/source/util/Collection.h b/source/util/Collection.h
index be7d778..c0dc6b8 100755
--- a/source/util/Collection.h
+++ b/source/util/Collection.h
@@ -19,22 +19,13 @@ class Sprite;
 
 /**
  * A collection of sprites, which can be put into different layers.
+ *
+ * Uses Sprite instead of sf::Drawable to also manage deleting objects.
  */
 class Collection : public sf::Drawable {
-// Public types.
-public:
-	/**
-	 * Determines in what order sprites are rendered, dynamics and actors should be on top.
-	 */
-	enum Level {
-		LEVEL_STATIC,
-		LEVEL_PARTICLE,
-		LEVEL_ACTOR
-	};
-
 // Public functions.
 public:
-	void insert(std::shared_ptr<Sprite> drawable, Level level);
+	void insert(std::shared_ptr<Sprite> drawable);
 	void remove(std::shared_ptr<Sprite> drawable);
 	void checkDelete();
 
@@ -44,7 +35,7 @@ private:
 
 // Private variables.
 private:
-	std::map<Level, std::vector<std::shared_ptr<Sprite> > > mDrawables;
+	std::map<Physical::Category, std::vector<std::shared_ptr<Sprite> > > mDrawables;
 };
 
 #endif /* DG_COLLECTION_H_ */