Invincibility:
While playing, hold Right then press B, X, X, X, B, X, Y, A.
Level Select:
While playing, hold Right then press X, X, B, Y, Y, B, X, A.
Weapon Upgrade:
While playing, hold Right then press B, B, Y, Y, X, X, X, A.
Skeleton Zeke:
Before the copyright screen at start-up, hold A, B, X, Y.
Invisible Zeke:
Before the copyright screen at start-up, hold A, B, L, R.
Features:
FAQ/Walkthrough v0.990.