That depends. Technically, all these categories of building can be fortified (Yes, palaces and mansions can be fortified)
Castles, palaces, mansions and chateaux are dwellings, their primary purpose was as homes for royalty, nobility and in later centuries, business magnates \ tycoons.
Forts on the other hand, have no real domestic component. They're strictly military bases.
"Castle" has been broadened to refer to any luxury residence or fortification, though it's most properly used to denote the heavily fortified residences of medieval Europe or feudal Japan. The term is also applicable to dwellings with some of the architectural characteristics often associated with medieval castles, like battlements and towers.
The term "chateau" can refer to any French or French style upper class home, whether it's a castle, palace, or mansion.
A country house in British English is a mansion located on a large piece of land in a rural area. The terms "manor house", "stately home" and fortificacthe" are synonyms.
"Palace" officially refers to an official residence of a country's ruler, or the leaders of the country's administrative divisions (AKA, states, provinces and their equivalents in other countries). But almost any grand house can be called a "palace",and architecturally speaking, there are private residences that are basically palaces.
A fort is a purely military fortification, containing no residential facilities except large open dorm rooms for the soldiers, as well as the auxiliary spaces, like mess hall and kitchen.
A citadel is a fortification located in or directly adjacent to a city, typically to domineer over the city and\or to serve as a final refuge for the denizens in case the city walls were breached. The citadel could also be the home of whoever was administering the city, in which case, the citadel was also a castle.
landmarks
is so broad that it can cover anything. Not very useful for our purposes, we're looking for more narrow tag.