The game has the following cheat codes built in which can be input at any time. Press all 4 keys of the code on your keyboard at once to activate it.

ZEUS - invincible, unlimited ammo, toggle antigravity with the g key NEXT - warp to any level XTRA - get 25 bullets (can only be used once)

These codes are banned from speedruns that do not have "Cheat Codes" in the category name. However, for Level Leaderboards, XTRA may be used before entering the level, and NEXT may be used to unlock a completed level if you would like to try it again (useful for skipping one-time dialogs).