Wolfenstein 2: New Colossus is rammed full of Easter eggs, callbacks to the original games, and neat little secrets for players to find. The big one, however, is a fully playable version of the original Wolfenstein 3D. Except, er, it's called Wolfstone 3-D in the actual game. Want to know how to find it? Check out the video above and you'll see the method. For those who'd rather read, though, here's a quick summary for you...
When you finally get full access to Eva's Hammer - your home base that you can wander round at your own leisure - you'll be able to play Wolfstone 3-D to your heart's content. You'll need to head to Club Kreisau, which is at the far end, and on the upper deck. You can either use the stairs in the middle of Eva's Hammer to get to the top level, or you can use the ones at the back of the canteen. If you've found the canteen, then Club Kreisau is right above it. Basically, it's quite easy to find.
Once you're in the club, look to the right of the bar, and you'll see an arcade machine and a pinball table. Just go and interact with the arcade and you'll have the option to play the entire game from scratch. Interestingly, you can save progress too, and unlock later levels as you shoot through it.
So, why is it called Wolfstone? There's actually a twist here - the game is identical to Wolfenstein 3D, except you play as a Nazi and you're hunting members of the Kriesau resistance. Their posters adorn the walls, and instead of portraits of Hitler decorating the halls, there are paintings of Terror Billy - our hero BJ Blaskowicz himself. It's a lovely touch, and one that makes perfect sense as the resistance fighters are using a captured German U-boat as a base.
There are plenty of other secrets and Easter eggs in the game too, so keep your eyes peeled for them.