From the Super Mario Wiki
The Front Entrance is the first interior room in Treacherous Mansion. In here, there are six portals to the Dark Age Exhibit, Ice Age Exhibit, Space Exhibit, Ancient Exhibit, Jungle Exhibit and either the Study or the Nautical Exhibit respectively. There's also a window Luigi can look out of into the Inner Courtyard, doors to the East Gallery and West Gallery and various gargoyles.
During E-1 Front-Door Key, Luigi can look through the window to the right of the door to see King Boo imprison one of the Toads in a painting. He cannot however enter the room at this time since a Polterpup has run off with the key.
In E-2 and all later missions, Luigi can use the Dark Light on the right gargoyle to reveal a chain. Pulling this chain will lower the stairs and reveal a safe that can be opened with the Strobulb. Inside the safe is a gem, but Luigi has to defeat a Strong Slammer to access it. The stairs can later be set to their default position by pulling the chain again.