First, create an empty game object by going to GameObject | Create Empty. From there, with the object selected, go to Inspector and set its name to GameController, and optionally, for neatness sake, set its Position to (0, 0, 0).

Underneath the name, you'll see the Tag property. Change it from Untagged to GameController.

Next, select Add Component | New Script, and once you are brought to the next menu, change the language to C# (C Sharp), and set the name of the script to GameController. Then press Enter or click on Create and Add.

Select the newly created script in the Assets folder of the Project tab, and move it to the Assets\Scripts folder. Go to MonoDevelop by double-clicking on the script file.

While our game does many things, the most important thing is the spawning of enemies, which is what we'll be adding in first. Let's create a variable to store our enemy.

Inside of the class definition, add the following variable and then save the file:

// Our enemy to spawn
public Transform enemy;

Now, we can set the enemy that we currently have in the scene, but we should instead make the enemy a prefab and use it. To do so, drag the enemy from Hierarchy into your Assets\Prefabs folder. Once we've created the prefab, we can remove the enemy object from our scene by deleting it.

Next, drag-and-drop the enemy prefab into the Enemy variable in the GameController component.

Next, go back into our GameController script by double-clicking it to go into MonoDevelop. Add the following additional variables to the component:

// We want to delay our code at certain times
public float timeBeforeSpawning = 1.5f;
public float timeBetweenEnemies = .25f;
public float timeBeforeWaves = 2.0f;

public   int enemiesPerWave = 10;
private int currentNumberOfEnemies = 0;

We now need a function to spawn enemies; let's call it SpawnEnemies. We don't want to spawn all the enemies at once. What we want is a steady stream of enemies to come to the player over the course of the game. However, in C#, to have a function pause the gameplay without having to stop the entire game, we need to use a coroutine that looks different from all the code that we've used so far.

Inside the Start method, add the following line:

StartCoroutine(SpawnEnemies());

Now that we are already using the function, let's add in the SpawnEnemies function as follows:

// Coroutine used to spawn enemies
IEnumerator SpawnEnemies()
{
  // Give the player time before we start the game
  yield return new WaitForSeconds(timeBeforeSpawning);

  // After timeBeforeSpawning has elapsed, we will enter this loop
  while(true)
  {
    // Don't spawn anything new until all the previous
    // wave's enemies are dead
    if(currentNumberOfEnemies <= 0) 
    {
      float randDirection;
      float randDistance;

      //Spawn 10 enemies in a random position
      for (int i = 0; i < enemiesPerWave; i++) 
      {
        // We want the enemies to be off screen
        // (Random.Range gives us a number between the 
        // first and second parameter)
        randDistance = Random.Range(10, 25);

        // Enemies can come from any direction
        randDirection = Random.Range(0, 360);

        // Using the distance and direction we set the position
        float posX = this.transform.position.x + (Mathf.Cos((randDirection) * Mathf.Deg2Rad) * randDistance);
        float posY = this.transform.position.y + (Mathf.Sin((randDirection) * Mathf.Deg2Rad) * randDistance);

        // Spawn the enemy and increment the number of 
        // enemies spawned 
        // (Instantiate Makes a clone of the first parameter 
        // and places it at the second with a rotation of 
        // the third.)
        Instantiate(enemy, new Vector3 (posX, posY, 0), this.transform.rotation);
        currentNumberOfEnemies++;
        yield return new WaitForSeconds(timeBetweenEnemies);
      }
    }
    // How much time to wait before checking if we need to 
    // spawn another wave
    yield return new WaitForSeconds(timeBeforeWaves);
  }
}

Now, when we destroy an enemy, we want to decrement the number of currentNumberOfEnemies, but it's a private variable, which means that it can only be changed inside the GameController class or one of the methods inside of the class. Simple enough? Now let's add a new function in our GameController class:

// Allows classes outside of GameController to say when we killed 
// an enemy.
public void KilledEnemy()
{
  currentNumberOfEnemies--;
}

Finally, let's go back into our EnemyBehaviour class. Inside the OnCollisionEnter2D function under the Destroy function call, add the following two lines:

GameController controller = GameObject.FindGameObjectWithTag("GameController").GetComponent("GameController") as GameController;
controller.KilledEnemy();

The preceding line gets the script GameController with the tag GameController. The as keyword casts the object to a GameController object. Casting basically tells the computer, "Even though the code says it's some class, I'm telling you that it's another one."

This will call the KilledEnemy function from GameController, onto which we set the GameController tag in step 2.

With all those changes, save both script files and run the game! Have a look at the following screenshot: