Added a bunch of stuff

Assets/Scripts/Background.cs

@@ -0,0 +1,43 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class Background : MonoBehaviour {
+	SpriteRenderer spriteRenderer;
+
+	void Awake() {
+		spriteRenderer = GetComponent<SpriteRenderer> ();
+
+		StartFade (0.1f);
+	}
+
+	void StartFade(float period) {
+		if (period <= 0) {
+			throw new System.ArgumentOutOfRangeException("StartFade argument must be a positive floating point value");
+		}
+		StartCoroutine (FadeCoroutine (period));
+	}
+
+	IEnumerator FadeCoroutine(float period) {
+		float absolute = 1.0f;
+		float degree = -0.01f;
+		const float lowerLimit = 0.8f;
+		const float upperLimit = 1.0f;
+
+		while(true) {
+			//wait
+			yield return new WaitForSeconds (period);
+
+			//fade routine
+			absolute += degree;
+
+			//flip fade direction
+			if (absolute <= lowerLimit || absolute >= upperLimit) {
+				degree = -degree;
+			}
+
+			//fade
+			spriteRenderer.color = new Color (absolute, absolute, absolute);
+		}
+	}
+}

Assets/Scripts/Background.cs.meta

@@ -0,0 +1,13 @@
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+guid: 72cfcd727b69cf543893b4d457e109e1
+timeCreated: 1523360847
+licenseType: Free
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
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Assets/Scripts/GameController.cs

@@ -1,11 +1,38 @@
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
+using UnityEngine.UI;
+using UnityEngine.SceneManagement;
 
 public class GameController : MonoBehaviour {
+	public Text timerText;
+	public Text scoreText;
+
+	void Awake() {
+		PersistentData.timer = 60;
+		PersistentData.score = 0;
+
+		StartCoroutine (DecreaseTick ());
+	}
+
 	void Update() {
 		if (Input.GetButtonDown ("Quit")) {
 			Application.Quit ();
 		}
+
+		if (PersistentData.timer <= 0) {
+			SceneManager.LoadScene ("gameover");
+		}
+
+		//update the texts
+		timerText.text = "Time Remaining: " + PersistentData.timer;
+		scoreText.text = "Score: " + PersistentData.score;
+	}
+
+	IEnumerator DecreaseTick() {
+		while (PersistentData.timer > 0) {
+			yield return new WaitForSeconds (1);
+			PersistentData.timer -= 1;
+		}
 	}
 }

Assets/Scripts/GameOverController.cs

@@ -0,0 +1,22 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+using UnityEngine.UI;
+using UnityEngine.SceneManagement;
+
+public class GameOverController : MonoBehaviour {
+	public Text scoreText;
+
+	void Update() {
+		if (Input.GetButtonDown ("Quit")) {
+			Application.Quit ();
+		}
+
+		if (Input.GetMouseButtonDown(0)) {
+			SceneManager.LoadScene ("gameplay");
+		}
+	
+		//update the texts
+		scoreText.text = "Score: " + PersistentData.score;
+	}
+}

Assets/Scripts/GameOverController.cs.meta

@@ -0,0 +1,13 @@
+fileFormatVersion: 2
+guid: 50cbb78cd78265f4fb87277d95bcd4ee
+timeCreated: 1523366783
+licenseType: Free
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
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Assets/Scripts/Monster.cs

@@ -3,9 +3,22 @@ using System.Collections.Generic;
 using UnityEngine;
 
 public class Monster : MonoBehaviour {
+	public int value;
+
+	void Awake() {
+		StartCoroutine (DestroySelfAfter (30f));
+	}
+
 	void OnMouseOver() {
 		if (Input.GetMouseButtonDown(0)) {
+			//TODO: play death sound
+			PersistentData.score += value;
 			Destroy (gameObject);
 		}
 	}
+
+	IEnumerator DestroySelfAfter(float seconds) {
+		yield return new WaitForSeconds (seconds);
+		Destroy (gameObject);
+	}
 }

Assets/Scripts/MonsterSpawner.cs

@@ -7,7 +7,11 @@ public class MonsterSpawner : MonoBehaviour {
 	public Vector2 motion;
 	public float delay;
 
+	AudioSource audioSource;
+
 	void Awake() {
+		audioSource = GetComponent<AudioSource> ();
+
 		StartCoroutine (SpawnMonster ());
 	}
 
@@ -17,6 +21,7 @@ public class MonsterSpawner : MonoBehaviour {
 			GameObject monster = Instantiate (monsterPrefab);
 			monster.transform.position = transform.position;
 			monster.GetComponent<Rigidbody2D> ().velocity = motion;
+			audioSource.Play ();
 		}
 	}
 }

Assets/Scripts/PersistentData.cs

@@ -0,0 +1,4 @@
+public static class PersistentData {
+	public static int timer;
+	public static int score;
+}

Assets/Scripts/PersistentData.cs.meta

@@ -0,0 +1,13 @@
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+timeCreated: 1523365472
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