Here below we have some fantastic diagrams of various castle's throughout history. Now, obviously, since there is no one single caste design, we have labeled diagrams for all sorts of variations. Feel free to comment on whether this page is complete enough.
It's a bit blurry in the References, so here is what they say:
1. Barbacan (also said as Barbican) 5. Artificial Mount
2. The Ditch or Meat 6. Wall of the Inner Ballium
3. Wall of the outer Ballium 7. Inner Ballium
4. Outer Ballium 8. Keep or Dungeon