Alternate Ending
On Level 9, when you reach the area where your enemy appears, press Down. Remember to not kill him. After talking to him, press Down. Go to the tunnel and destroy the computer by pressing Up and shooting it.

Secret Options
During gameplay, press Start. Then press A + B and a set of options will appear.

Easy Level Passwords
2 - DVYLWKVYYZ
3 - GRYLWKWVVN
4 - DRYLSRWVVZ
5 - GVZLSRSQVV
6 - DVZLBVSQQT
7 - GRZLBVBQQQ
8 - DRZLBVBQQT
9 - GVYNBVBQWC

Medium Level Passwords
2 - QVYLWKVYYC
3 - TRYLWKWVYY
4 - QRYLSRWVPT
5 - TRZLSRSQPS
6 - QVZLBVSQVN
7 - TRZLBVBQVL
8 - QRZLBVBQVN
9 - GVYNBVBQWC

Hard Level Passwords
2 - DLTLWKVYYC
3 - GNYLWKWVPP
4 - DNYLSRWVPT
5 - GLZLSRSVPW
6 - DLZLBVSVVB
7 - GNZLBVBQVL
8 - DNZLBVBQVN
9 - GLYNBVBQQD

Various Codes
Enter these codes at the cheats menu:
Unlimited Ammo - DLVTRKBWPS
Unlimited Lives - DLVTRKBLVS
Unlimited Energy - DLVTRKBNRG
Bird Mode - DLVTRKBBRD
Skip Levels - DLVTRKBLVL