A cheat

To enter any level without knowing the correct level code, enter the following 4 codes:
where XXXX is the level you want to enter; 0001 for level 1, 0008 for level 8 etc.
Note that the codes contain the number 0, not the character O.
The levels in expert mode have a different number, you must add 000A (hexadecimal) to it:
for example, 0005 is level 5 in beginner mode while 000F is level 5 in expert mode; expert level 6 would be 0010.