For practice purposes, cheats are allowed. Just type "pcenginebyneon" or "pcenginebzneon" (with english keyboard layout) in the main menu. There is no feedback, whether you did it correctly, so you need to enter a game and try it out. Once you're in the first level there are four available cheat codes:

"1" - full health "2" - finish level "3" - all keys "4" - level selector

Note that the numbers are not registered when typed with the numpad.

In the level selector the levels are in order as you play them: nature level 1-6 crocodile ("Krokodil", first boss) jungle level 1-6 snake ("Schlange", second boss) castle level 1-6 final boss ("Endgegner", dragon)

So basically you can enter every level and have unlimited health to explore everything. To turn off the cheats you have to restart the game. Please don't have cheats active while running the game.