Collect all five hidden keys in the indicated level to unlock the corresponding bonus behind the five-keyed doors:

Air Fleet: Air Saucer
Black Comet: Two Turrets
Central City: Air Saucer
Circus Park: Extra shooting gallery
Cosmic Fall: Switch that makes ring trails to the end of the level
Cryptic Castle: Shortcut
Death Ruins: Warp Portal
Digital Circuit: Warp Portal
Final Haunt: Turrets
Glyphic Canyon: Black Arm's Vehicle
GUN Fortress: Military Vehicle
Iron Jungle: Tank Cannon
Lava Shelter: Air Saucer
Lethal Highway: Large GUN Cannon
Lost Impact: GUN Vehicle
Lost Impact: Armored Car
Mad Matrix: Alternate path to red terminal
Prison Island: Tank Cannon
Sky Troops: Black Arm's Vehicle
Space Gadget: Warp Hole
The ARK: Three extra lives
The Doom: Open cage to shortcut to the end of the level
The Last Way: Warp Portal
Westopolis: Armored Car