During gameplay type these codes then press ENTER:
gmgodm  -   Invulnerability
gmpete  -   All powerups
gmkill  -   Enemies are easier to kill
gmbigh  -   You get a big head
gmborg  -   You become a cyborg
gmnorm  -   Turns you back to normal

Level Codes:
1.    DBDBBABA
2.    ADDCAADC
3.    ACBABBCC
4.    ADDDCACC
5.    DDBDBBCA
6.    CCDBCCDA
7.    AADBDDAC
8.    CADDCBCC
9.    ADAABADB
10.   BADDBBBC
11.   ABBDADDA
12.   DCADCAAC