Final answer:
The Forum of Trajan was a multifaceted public space in ancient Rome where the Basilica Ulpia, with its apses, primarily served as a courthouse for judicial proceedings.
Step-by-step explanation:
The Forum of Trajan in Rome, described as "a construction unique under the heavens", was a multifunctional public space used for various civic activities.
While the forum included various buildings and spaces such as libraries, a basilica, and the Column of Trajan, one of the main functions of the basilica within the forum was to serve as a courthouse.
The basilica, known as Basilica Ulpia, had apses where judges sat. Thus, these structures provided a space for judicial proceedings, where magistrates represented the authority of the emperor and issued judgments.
As such, the primary function of the Forum of Trajan was to serve as a central public area for civic, juridical, and ceremonial activities in Roman life.