SECRET #1
After starting the level and going through the corridor, turn right. After a walking a while, you'll see a red face on the wall, shoot it, and it'll open up.
Reward: Double Golden Pistol

SECRET #1
After exiting the starting building, you'll see that a part of the wall on the left is black. Go up to it and walk through it.
Reward: Armor Colete

BONUS:
This isn't counted as a secret, but it's still noteworthy.
in the area with the buildings, go up the hill, then jump on the roof of the closest building, and then on the roof of the right one.
Reward: Armor Colete

SECRET #1:
In the area with the gold key and glocks, go under the stairs you use to enter.

Reward: Eye of the blind,

SECRET #2:
In the secret #1 area, walk left. There, you will find another secret.

Reward:Shells, Second shotgun

SECRET #3:
In the corridor before the room with a structure of Mit Aia and 3 Brazillian flags, there is another photo of Mit Aia. Shoot it, and it will open up.

Reward: Rocket Launcher.

SECRET #1
In the building on the right, before entering, rocket jump onto the roof

Reward: Armor, Health

SECRET #2:
In the building on the left, while walking through the corridor unlocked by a key, jump into the box located over the lava pit.

Reward: Armor

SECRET #3:
Near the entrance to the exit, rocket jump onto the roof.

Reward: Health, Armor

SECRET #4:
From the roof of secret #3, jump onto the ledge on the left, and then drop down. (to exit this secret, you must rocket jump back, but the developer is adding another exit in the future.)

Reward: Health x4, Thunder snake

SECRET #1:
In the room with the silver key, shoot the wall in the upper right corner

Reward: Rocket Launcher.

SECRET #2:
Go back to the beginning and rocket jump onto the roof of the house you spawned in.

Reward: Rockets x16, Rocket launcher

SECRET #1:
In the corridor full of doors, the last door on the right can be opened.

Reward:Armor, Broken rocket launcher

SECRET #2:
In the area with the silver key, go behind the blue wall on the right.

Reward: Source code

SECRET #1:
Cross the pathway under the brighe, than jump into the lava and go left.

Reward: Ankh of power, health

BONUS:
If you go behind the alien, you will find a secret that isn't listed.

Reward: Source code

SECRET #2:
In the tunnels, press the red button with an exclamation mark above it, then quickly run away. The left wall will slide open, revealing the secret.

Reward: Rocket launcher, second shotgun

SECRET #1:
In the area where you get the gold key, after pressing the button the wall on the right will open, revealing a portal. The portal will take you higher up, walk right and into the building.

Reward: Rockets x2, Rocket launcher, Armor, Ankh of power

SECRET #1:
In the beginning area, shoot the rock formation on your right on the right side, and it will open up.

Reward: AK

SECRET #1:
After falling down the hole, there will be a button on the roof above you, shoot it.
The second button is located on the roof of the room with the gold door. Shoot it too.
After shooting the buttons, go back to the room you fell into, a wall should've opened there.

Reward: Rockets x2, Rocket launcher

SECRET #2:
In the water-filled room which you drop down into, there is a small ledge above the doorway to the room with the elevator. Rocket jump to it, there you will a door on the left that opens when shot at.

Reward: Source code

SECRET #3:
In the room with the guillotine, the wall on the left can be shot at to be opened.

Reward: Armor

SECRET #4:
In the room with a lava pit that you drop down into, there is a wall next to a lamp that can be opened by shooting it.

Reward: Ankh of power

SECRET #5:
In the room with the lava pit, there is a small circle full of lava that connects to the pit. Jump into it, and you will find the secret.

Reward: Rocket launcher

SECRET #6:
In the final room, next to the exit on the left wall is a stone texture. Interact with it to open it.

Reward: Rocket launcher, Thunder snake

SECRET 1:
On top of the roof in the blue building
Reward: Rocket launcher, Rockets
SECRET 2:
It is in this room here
Reward: Rocket Launcher
SECRET 3:
On this pillar here
Reward: Rockets
SECRET 4:
On the roof in the final arena
Reward: Boladao Invencivel, Source Code, Rockets, Shells, Bullets