The Sallis Mansion is first introduced in Who Could That Be at This Hour?. It is said to be the home of Dame Murphy Sallis a client of S. Theodora Markson and Lemony Snicket. Later in the book it is revealed that the mansion was actually abandoned by the Sallis Family.
It is first described by Lemony as being so huge that it resembled multiple mansions stuck together. Parts of it are castle-like, with long sloping towers, while other parts look like a tent, covering elaborate gardens. Other parts still resembled a museum, with long gaping windows and a sturdy front door. It was depicted as once having a pretty view, before the existence of The Clusterous Forest. There is a long set of brick stairs winding up to the aforementioned door. There is also a rather large set of double doors in front of the mansion's library. The furniture at the beginning of the book is wrong for the rooms and seems out of place, and the library contains very few books. These books are boring and ordinary. These books do not interest Lemony what so ever. The set up of each room is thrown together and not well thought out. The mantel of the fireplace is bare, although the Bombinating Beast was said to have been kept there. The windows are shielded by thick curtains, and are always kept latched. There is a small parlor above the library where Theodora believes a burglar must have broken in. The ceiling of the library is described as being red and blank. There is a thick hawser that runs all the way from the Mallahan Lighthouse to the mansion. There is a well on the property and also a guest cottage that used to be rented by a copy editor of the local newspaper, The Stain'd Lighthouse. There have also been many midnight badminton parties hosted on the grounds, which indicates the existence of a badminton court on the property. The property must also include a telephone because Theodora calls there to set up a plan. The entire mansion is confusing because of its conflicting architectural styles. The house is full of long hallways that ate empty but used to contain furniture like carpets. Towards the end of the book, the entire house has been stripped of its odd furniture. The floors are now bare. Potted plants are missing, and have left rungs where they used to be. At the end of these such hallways is a wide winding staircase with a wired lightbulb hanging above it. The upstairs hallways are lined with uninhabited empty rooms that are completely bare. The hastily arranged furniture meant to trick the viewer has disappeared. One room contains only a mattress and a few pillows, where someone evidently has been sleeping. There is a ventilated heating system throughout the whole house and the heater is located in the basement, next to the long empty living room, which overlooks the Clusterous Forest. The kitchen is empty and does not even contain a refrigerator or a stove. Behind a small white door, lies the mansion's enormous basement. There are a few windows in the basement on the far walls. The basement also contains a gray bricked water vat and a rusty level, that function through an underground spring. This is accessible by a pump. There also may be the remains of an armchair situated in the center of the basement. The entire empty mansion is prone to echoing, and had probably been empty a long time before it was used as a hideout.