All 3 Uses of

atrium
in
The Lost Hero

The upper floors made a ring of balconies around a huge central atrium, so they could see all the way down to the ground floor.†
Chpt 26

In the middle of the atrium a fountain sprayed water twenty feet into the air, changing color from red to yellow to blue.†
Chpt 26 *

Festus flew halfway up the atrium, then hurled the sun dragons into the pits they'd come from.†
Chpt 28

Definitions:

(1) (atrium as in: atrium of the building)
a large public room leading to other rooms--often skylit and containing plants and/or water features

or in ancient architecture:

a roofless area of a building--such as the center of a Roman house or an entry courtyard surrounded by roofed columns in an ancient Christian church
(2) (atrium as in: right atrium of the heart)
chamber from which blood enters the heart
(3) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much less commonly, atrium can refer to anatomical structures other than the heart.