Tarquinius Priscus, in tradition at least, began construction of the temple of Jupiter Optimus Maximus. The problem is, the south summit of the Capitoline isn't the best suited for monumental construction. The land had to be completely leveled. Tarquin enclosed the the area with a wall and filled it in to create an artificial, level platform on which to build the temple.

In 388, the Romans expanded this platform. Here's a bit of Platner's description:

Area Capitolina: * the open space in front of and around the temple of Jupiter Optimus Maximus on the south summit of the Capitoline (Suet. Cal. 22, 34; Gell. II.10.2; Sidon. Ep. I.7.8; Vell. II.3.2; Liv. XXV.3.14: area Capitolii), made by building retaining walls and substructures round the edge of the hill and levelling off the surface enclosed. The area was therefore in effect a built-up platform, part of which at least was contemporaneous with the foundations of the temple. It was enlarged in 388 B.C., and was regarded as a notable monument even at the beginning of the empire (Liv. VI.4.11). [<----------- this is your source]

Here is a map from RomeLab.