The Amphitheatrum Castrense was constructed by the emperor Elagabalus (r. 217-222 C.E.) as part of an imperial residential complex, the so-called Sessorian Palace, which also included the adjacent Circus Varianus.
This elliptical arena was modeled on the Colosseum and measured 88 x 77.8 m. It was constructed of brick-faced concrete. The exterior was articulated in three storeys. The first storey consisted of an arcade with attached semi-columns on travertine plinths and Corinthian capitals in brick. In the second storey arcade, the semi-columns were replaced by rectangular pilasters, also with travertine plinths and brick Corinthian capitals. In the attic storey, similar pilasters flanked square windows as at the Colosseum. Based on drawings of Dosio, Palladio and Peruzzi, the upper storey originally included travertine consoles to hold the supports for the arena’s awning, also similar to the Colosseum. The intimate scale of the Amphitheatrum Castrense suggests that it was used by the emperor for private spectacles and entertainment. Later in the 3rd century C.E. the amphitheater was incorporated into the new defensive walls, commonly known as the Aurelian Walls, constructed by the emperor Aurelian (r. 270-275 C.E.).
Pirro Ligorio was the first to correctly identify this amphitheater (Burns  25). Ligorio’s identification was based on a close reading of the regionary catalogues, which placed the amphitheater in Regio V (the Esquiline). These regionary catalogues (the Curiosum and Notia) were compiled in the 4th century C.E. and listed the most important monuments then existing in Rome’s 14 regions. In the 16th century they were attributed to the authors Publius Victor and Sextus Rufus. Prior to Ligorio, the Amphitheatrum Castrense had been incorrectly associated with the Amphitheater of Statilius Taurus, which was actually located not on the Esquiline, but in the Campus Martius. Ligorio comments on the controversy:
They would have it that the amphitheater that is at S. Croce in Gerusalemme is that of Statilius Taurus and they don’t let it be known that the Amphitheater of Statilius was of marble and this one is of brick and that that amphitheater was in the Campus Martius as Dio writes in his 40th book [Loeb Classical Library translation, 51.23.1]. Victor says in his texts, written in pen, that it was in the Circus Flaminius, which incorporated part of the Campus Martius. And this amphitheater of S. Croce was called by Publius “Castrense”.
Essi vogliono, che l’Amphitheatro, che è à santa Croce in Hierusalem sia quello di Tauro Statilio, & non avertiscono che l’Amphitheatro di Statilio era di marmo, & quest’è di mattoni: che quello era in Campo Martio (come scrive Dione al quarantesimo libro) & Vittore dice nelli suoi testi scritti à penna, che era nel Circo Flaminio, il quale abbraciava in se parte del Campo Marzio. Et questo di Santa Croce da Publio fu appellato Castrense. (P. Ligorio, Libro, f. 38r. in Burns  53 n. 47; see also Negri  28)
The imperial residence next to the amphitheater was used in the early 4th century C.E. by the empress Helena (246/50-330 C.E.), mother of Rome’s first Christian emperor Constantine (r. 306-337 C.E.). An atrium of this palace was later transformed into the church of S. Croce in Gerusalemme and the amphitheater was eventually annexed to its convent. The gardens of the convent are now contained within the remains of the amphitheater.
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