Press ~ during game play to display the console window and type "mlpdev" to enable cheat mode. Then, activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Add indicated number of lives - addlives
Restore health - health
Toggle enemy spawning - enemyspawn
Toggle master key - opensesame
Ending sequence - map end
Level select - map
Level skip - next
Restart current level - restart
Display current level name - name
Show available levels in directory - dir
Show help for basic cheats - help