To access the cheat entry screen, during gameplay press Up, Down, Up, Down. Activate the following cheats by pressing the corresponding sequences of buttons:

CHEAT FUNCTION - CHEAT CODE
Invisibility - UP, RIGHT, LEFT, DOWN(2), UP
Extra armor - RIGHT, LEFT, RIGHT, DOWN, UP, RIGHT
Decrease enemy accuracy - RIGHT, LEFT, RIGHT, DOWN, UP, RIGHT

You can fire two shots with the K98 German Rifle by going to the iron sights. Fire once, go out and then fire again.

Find weapons upgrades in the corresponding locations of the following missions:

MISSION - LOCATION
Mission 1 - Parachute onto the western cliff. It's next to the boxes, near the searchlight.
Mission 2 – It's on the counter of a small building at the entrance of the courtyard.
Mission 6 – Parachute into the western tower. It's on the top floor.
Mission 10 – Go inside the tunnel after the first 88. It's behind a box at the end of the first corridor.

Weapons can be upgraded in a stage if you find a certain power-up. Although you cannot carry the upgrade from one stage to another, it will benefit your weapons within that stage.

Mission 1: Weapon Upgrade
Parachute onto the western cliff and find the upgrade adjacent to the boxes, near the searchlight.

Mission 2: Weapon Upgrade
Look for this upgrade on the counter of a small building at the entrance of the courtyard.

Mission 6: Weapon Upgrade
Parachute into the western tower (the one emitting green gas) and find the upgrade on the top floor.

Mission 10: Weapon Upgrade
Inside the tunnel after the first 88, the upgrade is located behind a box at the end of the first corridor.

Unlock the following medals in your service record by performing the corresponding tasks:

MEDAL - HOW TO UNLOCK
Campaign Medal - Beat all levels in Campaign mode.
Bronze Stars - Beat a mission without dying.
Silver Stars - Beat Campaign mode without dying.
Oak Leaves - Earn any seven medals in Campaign mode.
Wings - Land in the designated drop zones.
Weapon Expert - Use all available weapons during a mission.
Marksman Award - Shoot a certain number of peoples in the head during a mission.
World War II - Earn all medals in Campaign mode.