It's a pretty amazing place with so much history, though most of it is not visible on its surface. The debris was covered with greenery to try and mask/beautify the area, and without knowing what the buildings were, you would have no idea that they were an American spy station.
It's quite a long walk to get there, but it's definitely worth it. This is such a different thing to visit; very typical of Berlin in a way: it's quite hidden, not very advertised, not exactly beautiful in any way, but full of history and you can spend hours there, wandering and wondering.
Rather than try to explain it, I'll just show you. Here are some pictures of Teufelsberg!