Etymology: Isengard and Helms Deep

[Inziladun]

I will give the usual disclaimer that I am far from being a Tolkien scholar on the level of others.

However, it's my understanding that Isengard was simply a translation, consistent with other place names in Rohan, from the Sindarin name Angrenost.

And what's so complicated about Helm's Deep? As explained in Unfinished Tales, it was

A deep gorge near the north-western end of Ered Nimrais, at the entrance to which was built the Hornburg; named after King Helm, who took refuge from his enemies there in the Long Winter of Third Age 2758-9.

With the myriad numbers of place names in various countries and languages, coincidence alone can easily account for similarities between 'real' locales and those in the mythos.