Serious Sam 3, BFE, Secrets: 05 Under the Iron Cloud

Parkour. Ammo (Sniper bullets +3)
Behind the pedestrian bridge you climb onto the rubble on the right hand side and onto the small ledge. From there you jump onto the next building's ledge. Crawl under the balcony and jump up against it on the other side to get the ammo.
Secret. C4 (+3)

Turn into the alley after the building in secret #1. There are some boxes on the right hand side. Jump over the wall to pick up C4.
Mini secret. Ammo (Electricity +50).

Secret. Ammo (Minigun bullets +100)

After getting off the bridge, turn right in-between the bridge and the building. There's a small garage with a car. Behind the car is the ammo.
Secret Cannon.

Get onto the roof of the garage and jump onto the ledge of the building. Follow it until you get to an AC unit. Jump onto that and across to the AC on the next building. From there you jump onto the top of the building to pick up the cannon.
Secret Laser

Continue past the garage and up to the bridge again and under it this time. You will find some barrels that allows you to crawl over the barb wired concrete barriers into a small area with a shipping container and the laser.

Picking up the laser triggers a kleer, so be prepared.

Secret. Ammo (Cannonballs +4)

From the bridge run straight forward until you pass the kiosk and blue container. On the left there's a small yard with another container with cannonballs on top of it. To get up there, you need to get onto the wall to the right of the gates. Follow the ledge until you can get to the container.
Useless secret area

From #7 jump back to the ledge and jump behind the end wall.

Mini secret. Armor (+10)

After the showdown at the underpass, you turn right and have another showdown. At the end of that road are some concrete barriers. On the left hand side there's a bush hiding a helmet.
Secret nuthouse. Health (+1).

From secret #9 you go to the right hand side of the barriers. Climb onto the wall from the hole in it, go through the crumbling building and out on the other side of the barriers.

Picking up the vial of health will spawn three rocket launching biomechanoids

Secret. Ammo (Cannonballs +4)

From secret #10, you turn right at the mosque and run to the end of the road. Turn left and head down the road. On the left hand side you should come to a small wall with a car at the end. Get onto the car and over the wall to pick up the cannonballs.
Parkour. Health (+1)

From secret #11, get back onto the wall and onto the thin ledge on the left hand side of the area with the ammo.

Walk along the thin ledge and jump across to the balcony to pick up the health vial.
Secret. Health (+100)

Once you get into the ruins, go to the right hand side. Follow the wall until you see the remains of a building going out from the wall. On the closest side of the ruined building, there is a pile of rubble hiding the health.