Enter the Museum and go to the Return Of Ishtar area. Quickly press Right, Left, Up, Down, and Circ. A new version of the Return of Ishtar game will begin.
Hold L1 + R1 when the Namco logo scrolls past while the game is loading. The FMV introduction sequence from the Genji and Heiki Clan will be displayed.
Go into the museum and from the information booth, go right and up the stairs. Now, turn to the left and you will see a room called the X-Room. Go in and hold L1 + L2 + R1 + R2 + Up together. With these held, press Triangle. You will hear a sound and the room will flash white for a second. A woman will appear. Move in front of her and press X. She will disappear and you will be left with the pedestal in the middle of the room and the words Assault Plus flashing above it. Access the pedestal and the arcade game will be the new Assault Plus game.