In Namco Museum 3, there are 2 additional versions of The Tower of Druaga, which are custom ROM-hacks of the arcade game made specifically for NamcoMuseum. They are both hidden away and unlocked through cryptic/hard means, with the latter basically being a cheat code in an already hard-to-find version of the game.

The first one, "Another Tower", is a much harder version of the game and is in a secret area of the museum. You have to hold L1 & R1, and rotate your dpad in a circle 3 times to get a pickaxe and unlock it.

The second one, which is unlocked by going to the Another Tower game's test mode, flipping Dip Switch #4, Going back to the game & entering a credit, and pressing X, Right, Circle, Down, Square, Left, R1, L1, R2, L2, allows you to access "Darkness Tower", an EXTREMELY hard version of the game. Like, "One of the treasures needed to beat the game requires you to open the disc cover on your console in order to get it" hard.

Thanks for the trivia thread OP, every time I try to start one nobody posts in it.