To access one of the bonus rooms at the end of the game, you must get there without killing any hostages, or allowing any hostages to be killed.
8/14/2001
Rogan Costume
Go through original mode, but at the begining select the 2.0 bullets and Double score items. Play through original mode, rescuing all civillians except for the one on level 4 (the guy the Zombie is holding on to). Then beat the game. You'll end up in a Secret room. In the trunk is a Rogan Costume and an Infinite Credits icon.
Beat all bosses in Boss Mode and get a ranking with each, to unlock Emperor and Fight All Bosses Mode
Goldman Costume
Beat Original Mode on any difficulty
Red Blood
Beat Original Mode with any difficulty (access after doing so by visiting the Options menu)
Access Training and Boss Modes
Beat the game in Original Mode
Infinite Continues
Beat the game in Training Mode and Boss Mode
Training Mission 9
Beat Training Mission 1 - 8 on a difficulty of 2 stars.
Training Mission 10
Beat Training Mission 1 - 9 on a difficulty of 3 stars.
Unlimited Credits (Original Mode)
Complete Boss Mode with 5-star marks for each boss, including Fight All Bosses Mode
Bruno Costume
In the room where you fight the Emperor, the last room to the left (shoot the door). A maid will give you it.
Display Score
Left, Left, Right, Right, Right, Left, Right, Start (on title screen)
1/3/2007
Score Display
Enter this code at the title screen, if entered correctly a "0" will appear in the upper-left corner of the black screen before the game starts. Now you can see your score during the game.