Monster Lair

Unlimited continues:
Hold Select and enter 68K as initials on the high score screen.

Overlapping characters:
Plug a TurboTap with a controller into port two. Select two player mode and pause game play. Replace the TurboTap and with a single controller. Resume game play with overlapping characters.

Continue from last game location:
Press Left, Right, Down, Up, then hold Select and press Left at the "Game Over" screen.

Sound test:
Hold I + II and press Run at the title screen.