1. The lift is surrounded by supplies. Before heading through the archway ahead, pick up everything you can, including cells, a BFG 9000, bullets, armor, rockets and grenades.
2. Don't miss these medkits either. You can climb the scaffolding to reach the top shelf.
3. Descend the stairs and find more medkits here. Watch the video to learn about the excavation site.
4. Approach the backpack behind the sarcophagus and the wall breaks apart. The backpack holds a BFG cell, plasma cells and bullets.
5. Follow the cavern, then this wall opens as you approach.
6. That's just not right. The wall closes behind you and the floor descends like an elevator. No turning back now.
7. Follow the twisting cavern. When you see these red stones, turn left.
8. Time for an easter egg. Press this stone with the id logo.
9. A wall panel slides open to your left, revealing this secret room.
10. Grab the id Software PDA from the center of the pentacle.
11. The PDA holds heartwarming messages from the dev team.
12. Backtrack to where you turned left a moment ago. This time turn right.
13. The wall before you crumbles, and a cutscene introduces Doom 3's final boss...
14. The Cyberdemon. Yikes!
15. Needless to say he's huge, and you want to keep your distance.
16. The arena is a large circular area where minions emerge from this center pit.
17. The Cyberdemon will only fall to the Soul Cube. Remember your Soul Cube is available every five frags and will replenish your health.
18. Focus on defeating the many Maggots and Imps to recharge your Soul Cube, then unleash it on the Cyberdemon.
19. Keep backing up, running around the circle of the arena as you fire at the Cyberdemon's minions.
20. In addition to running backwards, changing weapons and firing at the Wraiths, Imps and the Cyberdemon, you'll have to dodge the Cyberdemon's missiles as seen here.
21. Scarce supplies can be found, such as this medkit and armor.
22. More armor can be found here.
23. This is a tough battle for sure, but the Cyberdemon will eventually succumb to your Soul Cube.
24. Blood-splattered and beaten, the Cyberdemon falls.
26. The final cutscene plays. The Marines arrive from earth and you are rescued. Swann is dead. But what of Betruger?
27. He's living as a tongue inside a large flying demon-beast thing.
28. Is he the beast itself, or just its tongue? Is he happy with this arrangement? It's all a mystery and the boards are abuzz.
29. Lastly, the end credits. Game over.