observation deck

from the GNU version of the Collaborative International Dictionary of English.

• A room or platform at a high point in a tall building with a broad view of the surrounding area. It is often an outdoor platform, but is sometimes indoors in a room with large windows to accommodate viewing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An area of a tall building, often open to the sky, from which the public may look at the surrounding panorama
• noun A building (such as a hide) from which people may observe wildlife