Unlockable: Silenced Machinegun

To unlock a silenced machinegun, head to the in-game cheats menu and insert the following password: whattheymakeyougive.

Unlockable: Automatic Shotgun

To replace your assault shotgun with an automatic shotgun, head to the in-game cheats menu and insert the following password: alwaysanobjective.

Unlockable: More Concept Art

To unlock more concept art than is otherwise available to you, head to the in-game cheats menu and insert lastchancemarie.

Unlockable: Bonus Music Track

To unlock a bonus music track, head to the in-game cheats menu and insert jasonbourneisdead.

Hint: Even The Odds
After you get Castel's health down a certain amount he pulls out a knife. This gives Castel an edge in wrecking Jason Bourne's pretty face. You can disarm him though, by moving the fight over to the desk.

Once you are in front of the desk perform a takedown, and if Jason is in the right spot, he will stab Castel with a ball-point pen. Without the knife, Castel does a lot less damage. Note that you do need to do a takedown near the desk; any other location will allow Castel to keep his weapon.