Serious Sam: Next Encounter Cheat Codes for Gamecube

Bonus levels:
Successfully complete the game with the indicated number of "Gold" ranks to unlock the corresponding bonus level. To access them, press X at the level selection screen to display the "Lost Levels" level selection screen.

Ancient Rome - Praetorian Camp: 3 "Gold" ranks
Ancient Rome - The Forum Romanum: 5 "Gold" ranks
Ancient Rome - The Forum of Trajan: 7 "Gold" ranks
Ancient Rome - Addressing the Senate: 9 "Gold" ranks
Ancient Rome - Caesar's Sanctum: 12 "Gold" ranks
Feudal China - The Silk Road: 15 "Gold" ranks
Feudal China - The Gate of Supreme Harmony: 18 "Gold" ranks
Legendary Atlantis - The Steam Tower: 22 "Gold" ranks
Legendary Atlantis - The Geothermal Tunnels: 24 "Gold" ranks
Legendary Atlantis - The Corridors of Power: 26 "Gold" ranks
The Gallery: 31 "Gold" ranks