John Gill, on Jer 51:51

Though Babylon should mount up to heaven,.... Could the walls of it, which were very high, two hundred cubits high, as Herodotus (p) says, be carried up as high as heaven; or the towers of it, which were exceeding high, ten foot higher than the walls, as Curtius (q) says, likewise be raised to the same height: and though she should fortify the height of her strength: make her walls and towers as strong as they were high; unless this is to be understood particularly of the temple of Bel, in which was a solid tower, in length and thickness about six hundred and sixty feet; and upon this tower another; and so on to the number of eight, towers; and in the last of them a large temple, as the above historian (r) relates: but if these towers could have been piled up in a greater number, even so as to reach to heaven, it would have availed nothing against the God of heaven, to secure from his vengeance. The Targum is, "if Babylon should be built with buildings as high as heaven, and should fortify the strong holds on high:'' yet from me shall spoilers come, saith the Lord; the Medes and Persians, sent and commissioned by him, who would pull down and destroy her walls and towers, be they ever so high and strong. (p) L. 1. sive Clio, c. 178. (q) Hist. l. 5. c. 1. (r) Herodot. l. 1. c. 181.