A car without a roof is called a convertible
the headliner or roof
In the auto industry, it is called the "head liner".
Some cars have those removable tops on them or the tops of the cars go back so it has no roof, which are called convertibles . As the car can change from closed top (roof) to an open top (roof).
convertible Drop head Roadster (if it is a 2 seater)
Cupola has several meanings. It is a roof or ceiling in the form of a dome, a small roof structure sometimes protecting a skylight or ventilator on the roof of a building, or an observation dome on a passenger car or caboose of a railway train, or on a military vehicle. It also is a name for a vertical cylindrical blast furnace for melting iron for casting, and a gun turret on a warship.
A car that has no roof is merely roofless, but the name for such a car that has a retractable or removable roof is a convertible.
the tower on a castle is called a turret!