The original building having stood for nearly five hundred years, it began to exhibit signs of decay, and accordingly it was rebuilt upon the old foundations by Augustus, and dedicated by Tiberius. The podium or mass of rubble masonry therefore which we see beneath the three columns at the present day belongs to the time of the kings, while the columns themselves belong to the imperial period. Caligula used the temple as a vestibule to his palace on the Palatine Hill immediately behind. On the brow of that hill, separated only by the pavement of the modern street, projects a labyrinth of vaults, arches, and broken walls, a mighty maze of desolation without a plan, so interspersed with verdure and foliage that "it looks as much a landscape as a ruin." This is supposed to be the palace of Caligula; and its remains abundantly attest the extraordinary magnificence of this imperial domain, which contained all that was rich and rare from the golden East, from beyond the snowy Alps, and from Greece, the home of art. The substructions of this mighty ruin are truly astonishing; they are so vast, so massive, so enduring, that they seem as if built by giants. Concealed by modern houses built up against the foot of the palace, some of the remains of the famous bridge which Caligula threw obliquely over the Forum can be made out; two of the tall brick piers are visible above the houses, and in the gable of the outer house the spring of one of the arches can be distinctly seen. The bridge was constructed by Caligula for the purpose of connecting his palace with the Capitol, on the summit of which stood the magnificent Temple of Jupiter, so that, as he said himself, he might be able to converse conveniently with his colleague, the greatest of the gods! It is probable that it served more than one purpose; that it was used both as an aqueduct and a road for horses and chariots from the Palatine to the Capitol. Be this as it may, it must have been a stupendous structure, nearly a quarter of a mile long, and about a hundred feet high, striding over the whole diagonal of the Forum, with a double or triple tier of arches, like the remains of the Claudian aqueduct that spans the Campagna.