Play Japanese Classic Ninja Gaidens:
In the Options Menu, set the Text and Subtitles to Japanese. Then you can play the Japanese versions of the first 3 classic Ninja Gaiden games "Ninja Ryukenden".

Extra costume:
Beat the game on Normal or Hard, then hold L while choosing 'New Game'.

Extra costume 2:
Beat the game on Very Hard to get the Fiend costume.

Use starting costume:
Select New Game, hold L + R and press A. You'll use the blue costume for the whole game.

Plasma Saber:
Beat the game on Normal.

Plasma Saber 2:
Beat the game on Very Hard.

All FMV, Very Hard Difficulty:
Beat the game on Normal or Hard.

In-game reset:
Press Start + Back during play to return to the Main Menu.

Classic Ninja Gaiden game:
Give Maramusa 50 scarabs. He will give you a Ninja Gaiden disc, which you then can use in the arcade machine at Han's Bar.

Classic Ninja Gaiden 2 game:
First get the original Ninja Gaiden game from Maramusa. Then, go to the Clock Tower Plaza in Tairon. Climb up above the colored door that leads to Pleasure Street. Now shoot the face of the clock across the street using your bow and a treasure chest will appear. Enter the doorway across the street, climb to the top and grab the Ninja Gaiden 2 disc inside the chest. Then take that back to the arcade machine at Han's Bar.

Another option is to get a Master Ninja rank on every level on Normal.

Classic Ninja Gaiden 3 game:
You've unlocked two extra Ninja Gaiden games; why not go for three? After getting Ninja Gaiden 2, go back to the Peristyle Passage in the Aqueducts. There's a broken pillar there to climb. There used to be a scarab at the top, but now there's the disc for Ninja Gaiden 3.

A more difficult option is to get a Master Ninja rank on every level on Hard or Very Hard.

Level Codes for Classic Ninja Gaiden games:
Enter the following codes at the Continue screen in the Classic Ninja Gaiden menu:

The Secret Behind The Wooden Sword:
Wonder what that wooden sword is for? Well, it's not great until you upgrade it fully, at which time it becomes Unlabored Flawlessness, a giant sweeping weapon that does serious damage.

Windmill Shuriken:
Next to Han's Bar is an alley. You'll notice some tell-tale blue stripes right outside it. If you follow the blue stripes by wall running down the alley, you can jump up to a ledge containing the sweet Windmill Shuriken as well as a silver Xbox that heals you.

Unlimited Essence:
While in the Aquaduct chapter go to the room with all the big black bugs. Kill all the bugs until you're allowed to exit the room. Once out, save your game. Go back to the bugs room and use the Dabilaroh and use the full essence power up and kill about 50 bugs, exit save and continue this process until you have 99999 in essence.

Thanks to Revolution readers Martin Brodeur, Sevvy, Styles, meechi_15!