Easy "DEEPER!" Achievement...
	Various Steam Achievements...

In stage two, the jet mini-boss will drop a laser and a missile power-up upon dying. Pick up the missile power-up and don't die, or you will lose it. Keep playing until you face the pharaoh mini-boss in the same stage and beat him. Right after the fight, there will be a brain-looking thing on the right side of the screen. Shoot the dark door-looking thing coming out of the brain and it will open as an entrance to a secret level. You will also get "HAVE YOU SEEN IT?" achievement in the secret level.

Complete the indicated task to unlock the achievement.

Achievement	How to unlock
A NEW GAME MODE!	Arcade Mode unlocked!
DEEPER!	That's a secret! Don't tell anyone!
DO IT FOR HER!	Someone to care about!
FORGET IT!	Now we know!
GIT GUD!	Die 5 times without closing the game!
HAVE YOU SEEN IT?	I saw something flashing!
HERO!	...and no one will know!
IT'S OK!	It's a bullet hell afterall!
MANKIND LAST HOPE!	You better be ready!
MY GUN IS BIGGER!	Meet the first boss.
STILL GOING?	Survive 7 rounds in arcade mode!
THE GAME GOES ON!	Survive 5 rounds in arcade mode!
WHO IS HE?	We'll never know!
YOU ROCK FOR REAL!	Survive 10 rounds in arcade mode.
YOU ROCK!	Finally at level 2!