The monumental gate, situated next to the theater, led in Roman times to the center of Side. Even today, traffic to the historical part of the town passes this gate. An elegant building, adjacent to this gate and supported by the ramparts of the 4th century AD, was probably erected in honor of Emperor Vespasian.

The current height of the monumental gate is 13.5 meters, but in the ancient times the structure was much higher. The entablature above the gate once supported the statue representing, most probably, a quadriga or a two-wheeled wagon drawn by four horses.

In the 4th century AD the arch of the gate was partially walled up, and a smaller opening was left in the ramparts, allowing access to the city. Currently, some fragments of this wall are still standing, but its part was demolished in order to facilitate the entry into Side. Despite that the monumental gate is still a bottleneck of local communication, especially for the incoming coach parties.

The Vespasian monument got its current name because of the inscription placed on its architrave. This inscription informs that the monument was erected in honor of Emperor Vespasian. The structure is 5.4 meters high and 6.4 meters wide. It consists of a central, semi-oval niche and to aediculae (the niches that served as small shrines) placed on its sides. In the central niche once the statue of Emperor Vespasian stood, and the left and right aediculae were decorated with the statues of female figures. The aediculae had two columns at the front and two half-columns at the rear The columns were made in the Corinthian order.

The left-side aedicula of the monument is in much better state of preservation. All niches were topped with triangular pediments and the central pediment was decorated with the head of Medusa. The Vespasian monument played not only decorative and religious function, but also served as a fountain. Terracotta pipes brought water to this structure from the main aqueduct of the city.