Flavius Josephus, The Wars of the Jews
William Whiston, A.M., Ed.

[411] Now although the place where he built was greatly opposite to his purposes, yet did he so fully struggle with that difficulty, that the firmness of his building could not easily be conquered by the sea; and the beauty and ornament of the works were such, as though he had not had any difficulty in the operation; for when he had measured out as large a space as we have before mentioned, he let down stones into twenty fathom water, the greatest part of which were fifty feet in length, and nine in depth, and ten in breadth, and some still larger. But when the haven was filled up to that depth, he enlarged that wall which was thus already extant above the sea, till it was two hundred feet wide; one hundred of which had buildings before it, in order to break the force of the waves, whence it was called Procumatia, or the first breaker of the waves; but the rest of the space was under a stone wall that ran round it. On this wall were very large towers, the principal and most beautiful of which was called Drusium, from Drusus, who was son-in-law to Caesar.