Ancient battles are kind of hard for me to visualize. Really any battle is hard for me to visualize, but it's especially true with ancient ones. Luckily for me, the people at Creative Assembly who are behind the production of Total War: Rome II have put together this walkthrough of the battle of Teutoburg Forest.
If you don't already know, the battle of Teutoburg forest was a carefully planned and brilliantly executed ambush of 15,000 Roman soldiers by the barbarians of Germania. In the end, essentially not a single Roman soldier made it through Teutoburg forest alive. This awesome narrated walkthrough might help you understand why.