This article investigates an aristocratic domus located on the Arx, overlooking the better-known insula of the Aracoeli from the northern summit of the Capitoline hill. This domus was buried during the construction of the basilica of Santa Maria in Aracoeli in the thirteenth century and remained sealed until the 1980s. I have reconstructed its layout — at least three levels survive — relying on my architectural survey and on the Vittoriano archives. The original phase dates from the first century BC, but substantial restorations were made during the Flavian age, when the domus lost its fauces–atrium–tablinum pattern, and in the early third century AD, when it was expanded vertically by a deep cut into the tuff bank, received a new facade and was redecorated with frescoes. The domus of the Aracoeli must have been a residence of high status, and in the Severan age it was supplied by lead pipes bearing imperial stamps. Although the Capitoline was mostly occupied by public buildings, the Arx was a prestigious neighbourhood and not a sort of monumental acropolis. I discuss the development and the architectural design of the domus of the Aracoeli, including its underground residential spaces as well as its sculptural and painted decoration; finally, I examine the remodellings of the original atrium house from a socio-cultural point of view.