Map03: Second Floor UAC Force (Prodoomer)

This map starts with a short cutscene, after which it's advisable to stand back and let the other marines take down the nearby monsters so you aren't killed by friendly fire. Be warned that any other monsters that come into their line of fire will prompt the marines to open fire, often shooting through each other and you.

Secrets
1. After the initial fight with the cacodemons, the hallway splits north and south. Go north and jump up on the ledge at the corner. The computer console will raise to reveal a secret area.

2. When you get the yellow key, go down the short hallway to the south and check the wall for a secret door leading to some items and a single enemy.

3. In the blue key room, check the south wall for a secret door that leads to the large outdoor area, where there are several enemies and some items.

4. Once you have all 3 keys, search the north wall in the same hallway as secret #1 for a door that you can now open. You'll go through a few teleporters and end up in a large secret room with a couple of backpacks.

Rings
05 In the core room that undergoes a massive explosion upon your entry, rocket jump up to the central platform to find a ring.

Speedrun
The achievement GOTTA GO FAST is a pain to get, but it's possible. The cutscene will waste 30 seconds, so be prepared to run as soon as it's over. Use your rocket launcher and super shotgun to smack enemies out of the way, and use Invisibility, Turbo, and Freeze Time to make your life easier. Note that you do have to kill the 2 revenants past the yellow door for the reactor core door to open. The achievement will not count unless you finish in 2:15, not 2:20.