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View of the interior of the main chamber of the two-storey Market of Trajan. In the lower right is the open "courtyard" (Latin: aula) area that is now filled with modern museum displays. On this lower level, note the four entranceways into shops that opened out into the "courtyard." The posts and lintels of the entranceways are of marble. The walls of this structure are of poured concrete that is faced with Roman brick—opus testaceum. Note the square openings above the doors that let light into the shops and relieved pressure on the lintels. Above them notice the brick arches that also helped relieve the pressure.
In the upper portion of the image, the walkway of the second floor is clearly visible behind the modern railing—as are the shops that opened on to this walkway. The lower portions of the groin vaulting that covered the main hall are visible. These rest on projecting corbels that are on the top of opus testaceum columns
The Market of Trajan was designed by Apollodorus of Damascus, Trajan's architect, and built between A.D. 100–110. It is built into the Quirinal Hill, which had to be excavated away. It is preserved in places to six–storeys and contained 170 rooms!
The "market" had a variety of functions: shops for merchants, administrative offices for the government, and apartments. Today it houses a museum—Museu dei Fori Imperiali—in which artifacts from the various Imperial Fora are on display.