Built in red brick-faced concrete (opus latericium), the amphitheater followed the typical elliptical design seen in Roman arenas, though at a reduced scale. It was originally a free-standing structure but was later incorporated into the Aurelian Walls by Emperor Aurelian in the late 3rd century CE, during the fortification of Rome. This modification significantly altered its function, as much of the external facade was repurposed as part of the city’s defenses. Though heavily damaged over time, sections of the original structure, including arcades and portions of the seating area, remain visible today, offering valuable insights into its design and construction techniques.

The Amphitheatrum Castrense was likely constructed under the Severan dynasty, with scholars often attributing its completion to either Emperor Septimius Severus (reigned 193–211 CE) or his son Caracalla (reigned 211–217 CE). It was built near the imperial palace complex known as the Sessorium, which later became the residence of Emperor Elagabalus and then the Empress Helena, mother of Constantine the Great. The close proximity to the palace supports theories that the amphitheater served a more private, elite audience rather than the general Roman populace.

By the late 3rd century CE, Emperor Aurelian's massive fortification efforts resulted in the incorporation of the amphitheater’s outer wall into the Aurelian Walls. The lower arches were bricked up, transforming the structure into a defensive bastion. This adaptation ensured its continued use, albeit in a drastically different role, well into the medieval period. Over time, neglect and modifications led to the partial ruin of the amphitheater, though some of its original architectural features remain intact.

circa 145 CE

The Amphitheatrum Castrense featured a two-tiered arcade system, originally adorned with decorative pilasters and engaged Corinthian columns. Built primarily of brick-faced concrete, the structure exemplifies the Severan architectural style, emphasizing both practicality and aesthetic refinement. The elliptical arena measured approximately 88 meters in length and 75 meters in width, significantly smaller than the Colosseum, which suggests a more intimate setting.

The exterior comprised evenly spaced arches supported by engaged columns, resembling the stylistic elements of contemporary Severan architecture. Unlike the Colosseum, which used travertine extensively, the Amphitheatrum Castrense relied more on brick and concrete, a reflection of the economic and material considerations of the time. The seating arrangement (cavea) likely consisted of several tiers, though much of this is lost due to later modifications.

When Aurelian repurposed the structure as part of Rome’s fortifications, the open arches were closed off with masonry, and the amphitheater lost its original function as an entertainment venue. However, traces of the original decorative elements, including sections of cornices and pilasters, still survive.

Today, the remnants of the Amphitheatrum Castrense, partially visible near the Basilica of Santa Croce in Gerusalemme, provide valuable archaeological evidence of Rome’s evolving urban landscape, illustrating both its imperial grandeur and its strategic military adaptations.