Jet Set Willy II

Tip

The game has a hidden debug mode. While playing press simultaneously the SPACE and all four CURSOR keys. You will get prompted to enter a keyword, type CAVEMAN, and you can use various function keys.

F1 allows you to access any room
F2 allows single stepping (only really useful for debugging)
F3 gives infinite lives

Note: Pressing all keys simultaneous with an emulator on a PC is typically impossible, unless you have a special "anti-ghosting" gaming keyboard.