Talk:Highway to the Danger Room/@comment-24203813-20160211200537/@comment-25089892-20160212021305

Actually the difficulty of portals increases as you progress through the EZ. First 10 levels focus on one type of portal that summons only one zombie. Anther 10 levels now focus on spawning more portals per subwave and increasing the amount of zombies spawned. And so on and so on.