Cupola – An architectural feature usually crowning a roof, turret, or dome.  In usage today, "cupola" is a very generic term.  Cupolas can be round or square, opened or closed, occupied or unoccupied.

In earlier usage, a cupola was typically considered to be a circular or polygonal structure crowning a roof or even a turret.

A lantern – today lumped under the category of cupola – was defined as a small structure, again crowning a roof, but with windows all around to let light into the spaces below.

Another term – belvedere – refers a rooftop structure, or top level of a tower, from which one can stand and look out.  If a bell is hung in this space it is considered a belfry.