Why do 'fast' and 'fastness' mean the same thing? [closed]

Question

...to the north the impenetrable forests of Gurgan and the Elburz range, to the south the fastnesses of Persia itself, a province which Artaxerxes III, contemporary of Philip, had never visited during his reign...

I've never seen 'fastness' used like in the quote, but what's meant seems to be (approximately) the same as 'fast'. Dictionaries corroborate this (edit: this was mistaken; actually, 'not exactly', or 'no they don't')

What is the difference between 'fast' and 'fastness'? And, come to think of it, what about 'fort' and 'fortress'? Is there a subtle rule here?

(edit): I mean 'fast' in this sense: https://en.wiktionary.org/wiki/fast#:~:text=prypyn%C3%BDty%20vohon%CA%B9)-,Etymology%202%5Bedit%5D,-From%20Middle%20English

the sense "fortress, enclosure"

(edit 2): this may not be a strictly 'English', ie Modern English, question. I may close it soon.

(edit 3): I frankly don't know why I thought that 'fast' as a noun meant something like 'fortified/secure area'. Maybe I once saw it intended an adjective in a grammatically ambiguous context. In any case, I've answered my question.

Answer 1

They don't mean the same thing. In this context, one is a noun and the other an adjective. A "fastness" is a thing which is fast.

"Fast" in this context means secure or fixed, as in "hold fast", "held fast", and describing attachment hardware as "fasteners".

According to dictionary.com, "fastness" can also relate to velocity, but that isn't how it's being used in the context of the quotation.

[Edit : I didn't spell it out at first, as I was trying to avoid stepping on user888379's toes, but it looks like it's worth adding that the quotation in the question was referring to Persia as being secure (from invasion) as it was heavily fortified.]

Answer 2

I'm pretty sure the fastness here is the second definition in the link:

2 a : a fortified or secure place b : a remote and secluded place