Type in the following codes during gameplay:

Level-Code
2 - AAJIHGDDC
3 - CGGHDGGDG
4 - HDICICCII
5 - IDHEHDGCC
6 - IJIIDIHEC
7 - CFDFEFEFJ
8 - JIIJIIIIC
9 - AAAABAAAA
10 - CCGDGBBBB
11 - HHIAAAJJIG
12 - GGDDJJHFD
13 - JIECBFGFF
14 - HGGEDDCCB
15 - HHHGFGDCC
16 to 20 - IHHDCHGHFF

Level skip
Type tuleby or tulebz during game play.

Invincibility
Find the computer terminal in the south part of level 2. Enter ALIENS ARE FAGGOTS at that terminal.