A building with a round roof, like the Capitol building in Washington DC or Saint Paul's cathedral in London.
It is called the Domed Rotunda
rotunda
Rotundarotunda
Astrodome in Houston Texas
its domed roof was partially modeled on classical roman building
Taj Mahal
Domed building rotunda
Rotunda
temple?
A domed hall is a rotunda.
Pantheon. (Which is actually a Greek word for the Roman building.)
The Radcliffe Camera, a circular domed building which is part of the university library. The university itself has many iconic buildings.