Part 1

Leave the shack and turn left. A Gnaar will attack. Use E or the left mouse button (PC controls) to rip its eye out.

Head to the edge of the building and throw the eye towards the green light to receive a shotgun. Use it to kill another Gnaar. (Secret 1/17)

Go around the next corner and jump to the building across the way. Behind a small shack, find a secret sledgehammer (2/17).

Jump down the building. Turn right and go to the next roof. Turn back to the starting building and jump to the grated balcony.

Turn around and jump to the second balcony from the top to collect secret shotgun shells (3/17).

Jump to the next grated balcony, then to ground level.

Facing the starting building, turn left and proceed. In an alley on the left, find a small green thing. Grab the secret shotgun shells.

Turn right and kill a Beheaded Rocketeer (4/17).

Head out of the alley and turn left to emerge on a street. Kill an attacking Gnaar and head forward.

Go around the blue dumpster to the right to find wooden crates. Jump on them, then to the dumpster, then to a red object, and finally to a balcony for some secret armor (5/17).

Jump down and head to the end of the street with strange writing. A giant alien will pass by.

Turn right into an alley, grabbing two red health pills. Go forward, kill a Gnaar that emerges from the wall, and grab more health.

Enter the hole the Gnaar made to come out on a street. Kill the Gnaars that attack.

At the end of this street, behind a wall, find some health.

Enter an alley on the right side of the street. A Gnaar attacks. Find health pills in the garages to the right and at the end.

Facing where you came from, turn left, then at the end go right. Take the single health pill for a mini-secret (6/17).

Exit and go to the end of the street. Turn left. A garage to the left holds more health.

Enter a hole in the brick wall.