Parker, John Henry: The archaeology of Rome (1,text): I. The primitive fortifications (Oxford [u.a.], 1874)

Section IV.

The Capitoline Hill, or Tarpeian Rock, originally called Mons
Saturnius, [added a.u.c. c. 33 (?), b.c. c. 72o(?).J
The hill now called the Capitol was originally a promontory
projecting from the high land of the Quirinal, or at least divided
from it by a shallow valley. This was afterwards deepened, so that
the two hills were divided by a deep foss, through which a road
passed. It was again enlarged by Trajan to make his forum, as
recorded in the inscription on the base of his column, which marks
the height of the hill cut away in this part. On the edge of the
Quirinal, immediately on the other side of this great trench, stand
the remains of the Torre dei Milizie, and near it the Torre dei
Conti, built upon an ancient wall of Etruscan character of the time
of the Kings.
The Capitolium Vetus* is recorded in the Regionary Catalogue
as situated in Regio VI., which includes the Quirinal, and it seems
not improbable that this was the Arx of the Sabines under King
Tatius, the present Capitoline Hill being either not then separated,
or being a detached fortress, afterwards made into the Capitol of
the united people under the general name of Romans.
“Being informed of this disposition of their forces, Tatius, king of the Sabines,
. . . before the sun arose, intrenched himself in the plain between the Quirinal
and the Capitol r.”
“In the year of Rome 364, six years after the capture of Veii, Camillus having
been driven into exile, Rome itself was taken by the Gauls, with the exception
only of the Capitoline Hill, then the arx or citadel of the city. Camillus being
recalled and made Dictator, causes a young man to be smuggled into the Capitol
to give information to the garrison of approaching succour; he contrived to enter
by way of the Tiber. The Gauls being informed of this, attacked the arx, but
were repulsed by Marcus Manlius1.”

The Tarpeian rock must always have been to a great extent
a natural fortress. The material of which it is formed is peperino,
or part of peperino and part of hard tufa; in any case it is a hard
stone, rough and not easy to work, but very durable, and the natural
formation is likely to have been in vertical cliffs, perhaps made more
so by scarping in places. The top of the rock is not level, but lower
in the centre, and an inclined plane {clivus) led up to this part on

x The Capitolium is also given in the
same series in Regio VIII., or Forum
Romanum, in which Regio the Capito-
line Hill stands.

r Dionj's. Ant., ii. 38.
z Dionys. Hal. Antiq., lib. xiii. ap.
fragmenta, p. 34; Livii Hist., lib. v.
c. 47.