12 double--that is, 12 in the outer boundary built over the
roads where they crossed the Pomœrium, corresponding
with twelve in the line of Servius, thus making in all,--
37, as mentioned by Pliny.
Of the twelve gates in the outer boundary, eight still remaining are
composed of work of an earlier date than the Wall of Aurelian. The
twelve may thus be named: the four gates of the Prætorian camp (two of
these partially remain, showing brick-work of Tiberius), Porta Chiusa
or Viminalis, Tiburtina, Esquilina now Maggiore, Lateranensis, Latina,
Appia, Ardeatina, Ostiensis.
Pliny (iii. 9) tells us that Tarquinius Superbus raised an outer agger
on the eastern side of Rome. Traces of this still remain, and the tufa
stones have been reused in Aurelian's work, whilst the Porta Chiusa is
partly formed on the inside of these blocks, and was probably the work
of the last of the Tarquins. The Porta S. Lorenzo, or Tiburtina, bears
inscriptions of Augustus and Vespasian; Porta Maggiore, of Claudius,
Vespasian, and Titus; whilst Porta Lateranensis and Porta Ardeatina
were undoubtedly built, as the construction shows, by Nero; and the
inner arch of the Porta S. Paolo, or Ostiensis, is of the time of
Claudius.
Tacitus (xii. 23) says: "The limits of the city were enlarged by
Claudius. The right of directing that business was, by ancient usage,
vested in all such as extended the boundaries of the empire. The
right, however, had not been exercised by any of the Roman commanders
(Sylla and Augustus excepted), though remote and powerful nations had
been subdued by their victorious arms."
"With regard to the enlargement made by Claudius, the curious may be
easily satisfied, as the public records contain an exact description"
(xii. 24).