The grand hallway is where you start off in Mission 1, in Mission 8, you're on top of a large platform, and after you jump over and into the courtyard through the shop room, you will fight Phantom for the last time, then you will find a trident, and you see that machine with three holes, put the trident in there, and the gate will open, but you have to get through quickly, unless you're on Easy Auto., the gate will begin to shut back down and if it does, you'll have to reuse the trident all over again.