The Arch of Septimius Severus bestrides the most important road in Rome, the Via Sacra, which runs across the Roman Forum. The most significant sacred and political ceremonies of the ancient city took place here and the most evocative of these was certainly the Triumphal Procession. The Lapis Niger, a large piece of black marble, marks an underground space which was probably a holy area dedicated to Romulus. The material is different from the stone found in the rest of the forum.

After the Arch and Lapis Niger, you come to the political area of the Forum, the assembly area. This space was used in the archaic era for political and judicial assemblies but it was drastically reduced in size to make room for the forum built by Julius Caesar. Just beyond is the Curia building. Caesar destroyed the original in order to create his forum and built the Curia Julia instead. This in turn was reconstructed in the 3rd century AD by the Emperor Diocletian and is the building you can see today.
The Curia has come down to us in an almost intact state because it was transformed into a church, Saint Alessandro, in the 7th century. You can still see traces of frescos decorating the internal niches and the tombs created in the external walls.
The building is a large rectangular cube and its height, width, and depth correspond to the proportions Vitruvius calculated to obtain perfect acoustics. Around 300 senators had sitting places on the three wide steps that ran along the length of the building, while the leader of the senate sat upon a podium in the center of the back wall. The floor paving is composed of the original marble pieces but was reconstructed in the era of Diocletian.