@5ALIEN BREED 2@1

Level codes:

2:  353828
3:  108383
4:  370101
5:  982822
6:  847464
7:  737373
8:  928112
9:  267364
10: 193831
11: 090921
12: 309383
13: 101221
14: 103992
15: 998112
16: 125332
17: 091233

Also try the following cheat codes for various effects:

736353            START WITH 50,000 CREDITS
098654            START WITH 10 LIVES
378829            START WITH 50 KEYS
243433            SKIP LEVELS BY PRESSING N IN PLAY