These are mentioned also in Book of Lost
Tales, Part Two... but without location.
It's said in the Appendix "The original test of
Tuor A had Babylon, Nineveh, Troy, and (probably) Rome."
these were later changed to more "
Middle-Earth" nomenclature.
I believe it refers to great
battles and destructions, and the reference indicates the terrror Melko wrought on Amon Gwareth, among the Gnomes.
From the text in The Book of Lost Tales:
"Glory dwelt in that
city of
Gondolin of the Seven Names, and its ruin was the most dread of all the sacks of
cities upon the face of Earth.
Nor Bablon, nor Ninwi, nor the towers of Trui, nor all the many takings of Rûm that is greatest among
Men, saw such terror as fell that day upon Amon Gwareth in the kindred of the Gnomes; and this is esteemed the worst work that Melko has yet thought of in the world." (
Tolkien, J.R.R.. The Book Of Lost Tales, Part Two (History of Middle-Earth 2) (p. 196). HarperCollins. Kindle Edition.)
I'll continue looking... so far I don't think locations are given anywhere. Just references to great, dreadful
events.
Addendum: No reference found in any of his other works, nor the Atlas of Middle-Earth, nor other resources about his works I have available electronically. There are 3 volumes of HoME not available electronically... I'll check those at some point. Outlook remains unchanged - additional references unlikely. (But there are
Giants among us...and they may have more.)