To unlock a hidden level, destroy all statues in the "Valley Of The Kings" level.

At the main menu, hold LT + RT and press LB, RB, LB(2), RB to unlock the "Cheat" menu with the following options:

Kill All
Give All
Max Health
Disable HUD
Chapter Select

ACHIEVEMENT - HOW TO UNLOCK
Serious Beginner (10 points) - Beat any level in single player.
Game Master (20 points) - Beat the game.
Serious Sam (30 points) - Beat the game in single player on serious difficulty.
I am Invincible (10 points) - Beat the game in single player without firing a single bullet or shell.
Serious Run (15 points) - Beat the game in single player by beating estimated time on each level.
Metropolis King (20 points) - Beat Metropolis level in single player on serious difficulty without loading.
Cannon Expert (5 points) - Pierce at least 10 enemies with a single cannonball.
Perfect Kill (10 points) - Kill all enemies in Karnak in single player.
Look, It's a Secret (30 points) - Find at least 50 secrets in single player.
Co-op Beginner (5 points) - Beat any level in co-op.
Co-op Hippie (10 points) - Beat The Great Pyramid in co-op with blood option set to hippie.
Serious Co-op (35 points) - Beat a co-op game on serious difficulty and extra enemy strength on 400%.