To enter a code, simply go to the title screen, and select "level code".

Code	Effect
L1. 426 - 819	Human Level No. 1
L1. 451 - 577	Alien Level No. 1
L10. 083 - 137	Human Level No. 10
L10. 157 - 037	Alien Level No. 10
L11. 412 - 320	Human Level No. 11
L11. 486 - 386	Alien Level No. 11
L12. 215 - 166	Human Level No. 12
L12. 486 - 574	Alien Level No. 12
L2. 089 - 825	Human Level No. 2
L2. 344 - 883	Alien Level No. 2
L3. 147 - 589	Human Level No. 3
L3. 287 - 435	Alien Level No. 3
L4. 090 - 297	Alien Level No. 4
L4. 279 - 383	Human Level No. 4
L5. 090 - 493	Alien Level No. 5
L5. 353 - 583	Human Level No. 5
L6. 024 - 813	Human Level No. 6
L6. 485 - 382	Alien Level No. 6
L7. 156 - 629	Alien Level No. 7
L7. 411 - 536	Human Level No. 7
L8. 049 - 868	Human Level No. 8
L8. 091 - 017	Alien Level No. 8
L9. 288 - 611	Alien Level No. 9
L9. 379 - 023	Human Level No. 9