From the Super Mario Wiki
The Armory (Japanese: 甲冑の部屋 Armor and Helmet Room) is a room cleared in the fourth Area of Luigi's Mansion.
When Luigi enters the Armory, he opens several chests, revealing two Purple Punchers. He also investigates the suits of armor, revealing a Blue Twirler, a Red Grabbing Ghost, a Garbage Can Ghost, and a Gold Ghost. After Luigi captures the ghosts, he receives the Key to the Pipe Room.
The Armory contains many suits of armor, which hit Luigi with their clubs if he stands in front of them. The room also contains five chests, three of which contain treasure. Many crates can be found in the room as well. A covered mirror sits on the north wall.