Sound test:
Press R1(4), R2(6) at the title screen. The sound of a scream will confirm correct code entry. A "Sound Test" option will now appear on the "Mode Select" screen.

Extra Ark:
Entering Astarte as a name for an extra 2800 Ark.

Extra traps:
Each of the traps require you to beat the game repeatedly and attain all four endings. Note that you still have to have enough Ark to make these traps.

Evil Upper and Evil Stomp:
To get the Evil Upper and Evil Stomp, play through the game and get the first ending. Getting the first ending requires that you to kill everyone in Chapter 17 and Chapter 21 (Two Wrongs Equal One Right).

Magic Sac:
While not doing any damage, the Magic Sac is a fun weapon to use since it immobilizes an enemy, and allows you to push them anywhere. The Magic Sac is only available after you get the second ending, requiring you to let at least one person escape in Chapter 17, and killing Keith in Chapter 24 (Shared Wounds).

Ardebaran Mask:
Although it looks rather foolish, the Ardebaran Mask does a lot of damage, and it makes the victim walk straight. To get it, you need to get the third ending, where in Chapter 17, let at least one person escape, and in Chapter 24, let Keith live (slightly damage him).

Evil Kick:
A very powerful trap that sends a victim flying across the room, the Evil Kick is gained when you get the fourth ending. The fourth ending is similar to the first ending, except you in Chapter 21, you let one person escape.

Suezo:
The eyeball monster from Monster Rancher can be found in the game. Although there is no animation, the Suezo is a very powerful trap. To get it, you need to have all four endings saved on one game.