The roof
A car without a roof is called a convertible
Sun roof
the headliner or roof
In the auto industry, it is called the "head liner".
A Turret
window alcove
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.
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)
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!