The caverns of Helm’s Deep, translated “Glittering Caves” (LotR/548) or “Glittering Cavern” (RC/421). It is a compound of aglar “brilliance” and rond “vault, high roofed cavern” (RC/421; SA/aglar, rond).
Conceptual Development: This name first appeared as N. Aglarond “Caves of Splendour” in Lord of the Ring drafts from the 1940s (WR/76), probably already with the etymology given above.
aglar (“glory, brilliance”) + rond (“dome-roofed hall”)