12 Guardian of Time

1. Secret. Ammo (Devastator shells +20). After the massive wave of rocketeers, you get to a fork in the road. Go straight instead of left towards the exit. At the end of this branch, up high, you'll find a box. Shoot it to get the ammo.
See this video for secret 1:


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2. Secret. Ammo (Sniper rifle bullets +3). When you get to the large valley, head straight down towards a temple like building. Inside it you'll find a hole in the roof, covered by a box. Shoot the box to get the ammo.
3. Secret. Ammo (Electricity +200). On the left side of the valley, there's a large stone arch. Behind the left "leg" you'll find the ammo.
See this video for secrets 2 and 3:


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4. Secret. Ammo (Devastator shells +20). After the big fight with spiders and just before the massive wave of bulls and Khnums, you'll find a large collection of supplies at the middle of a large double archway, including shells and some health on two boxes. Destroy the box under the shells to collect the ammo.
See this video for secret 4: