hey,

Do you have all those levels in different scenes. if you have it as different scenes means first add all those scenes when you build the project. when you hit build it shows a button name as ADD CURRENT.first you just hit it and add all those levels of your project.after you add this it'll appear like this,

Scene1 Name - 0 Scene2 Name - 1 Scene3 Name - 2

you'll get like this,if you want to move 2nd scene after killing those enemies means just use Application.LoadLevel(1), and move 3rd scene means use Application.LoadLevel(2).

see this link http://unity3d.com/support/documentation/ScriptReference/Application.LoadLevel.html

if you're not using different scene you cant use Application.LoadLevel.

Simple and enjoy.

         // Loads the level with index 0

           Application.LoadLevel (0);




          // Load the level named "HighScore".

           Application.LoadLevel ("HighScore");

This function loads level by its index. You can see the indices of all levels using the File->Build Settings... menu in Unity. Before you can load a level you have to add it to the list of levels used in the game. Use File->Build Settings... in Unity and add the levels you need to the level list there.

Well, in order to detect when all the enemies are destroyed (in order to load the next level), you'll need to keep some kind of counter to keep track of how many enemies are left. That way, when the counter reaches zero, you know you have no enemies left, and you can advance to the next level.

I recently gave an answer to a very similar question here. Since the script is almost exactly what you need, just replace "cannon" with "enemy" and you should be good to go. Oh, and you'll also need a way to keep track of what level you're on, so you that you can advance to the next one.