A clock tower is located on the roof of a building.
An otherwise ordinary roof and chimney are offset by an ornate tower resting in the middle of the roof that features ornamental work, a walkway, a weather vane, and a clock.    
A clock tower that has two clocks sits in the middle of a sky light.
A large building with a tower that has a clock on it.
This steeple with a clock sits on an old building