Rupert Giles's apartment is, as its name suggests, the home of Rupert Giles in Sunnydale. While the interior is seen a few times before and after the fourth season, it is featured most prominently in the fourth season, as it takes the place of the Sunnydale High School library after the destruction of Sunnydale High as the Scooby Gang's primary headquarters. Post-season four, it is replaced with the Magic Box and then Buffy's house.
It is never seen again after "Forever"; seasons six and seven show Giles living in the Summers household.