The upper wall, the one along the Corso, can not be traced farther than
the Piazza Garibaldi, in front of the Cathedral. It has been a mistake
to consider this a high wall. It was built simply to level up with the
Corso terrace, partly to give more space on the terrace, partly to make
room for a road which ran across the city here between two gates no
longer in existence. But more especially was it built to be the lower
support for a gigantic water reservoir which extends under nearly the
whole width of this terrace from about Corso Pierluigi No. 88 almost to
the Cathedral.[43] The four sides of this great reservoir are also of
opus quadratum laid header and stretcher.

The lower wall, the real town wall, is a wall only in appearance, for it
has but one thickness of blocks, set header and stretcher in a mass of
solid concrete.[44] This wall makes very clear the impregnability of
even the lower part of Praeneste, for the wall not only occupies a good
position, but is really a double line of defense. There are here two
walls, one above the other, the upper one nineteen feet back of the
lower, thus leaving a terrace of that width.[45] At the east, instead of
the lower solid wall of opus quadratum, there is a series of fine tufa
arches built to serve as a substructure for something. It is to be
remembered again that between the arches on the east and the solid wall
on the west is a stretch of 200 feet of opus incertum, and a space where
there is no wall at all. This lower wall of Praeneste occupies the same
line as the ancient wall and escarpment, but the most of what survives
was restored in Sulla's time. The opus quadratum is exactly the same
style as that in the Tabularium in Rome.

Now, no one could see the width of the terrace above the lower wall,
without thinking that so great a width was unnecessary unless it was to