I disassembled the executable with IDA.
Somewhere in the executable there are a bunch of bytes that are either 0 or 1, depending on whether a key on the keyboard is pressed or not.
Esc is the first key in this bunch, ! or 1 is the second and F6 is the 40th (in hex). Not every key is used in the game, but I found out F6 is used.
I also found out the game does something when you start the game with the command line argument asp and you press ALT + F6 at the same time.
I used the DOSBox debugger to set a breakpoint at the start of the piece of code that checks whether F6 is pressed or not. Then I played the game and started pressing ALT + F6. Suddenly the debugger popped up. I pressed F5 to continue the execution and then I was playing the demo level.
Or something like that. It's more than
6 years ago that I discovered this cheat.