9 Full Video Games You Can Unlock Inside Other Games

6. Ninja Gaiden NES & Ninja Gaiden Arcade (Inside Ninja Gaiden Black)

A brutally hard action series for multiple generations of players, either you were introduced to Ninja Gaiden through its 80s incarnations, or you played the brilliant - and infuriating - remakes.

Both are stellar, but what many players missed while they were hurling controllers out windows and snapping whatever they could get their hands on, was that the original Ninja Gaiden titles are unlockable inside the 2000s editions.

Where 2004's Xbox release let you play the NES version of the game if you collect 50 Scarabs, Ninja Gaiden Black went one better, including the very first arcade version of Ninja Gaiden, which is a completely different title altogether.

Being a far easier side-scrolling beat 'em up rather than a tight action platformer, you can flip around enemies, launch them into the scenery and pull off a much wider moveset than what was in the NES edition.

Combined - and taken alongside the stellar remake by Team Ninja, it's an essential package.