Crossword Genius

Stronghold must be built with speed? (8)

Ross

I believe the answer is:

fastness

I'm a little stuck... Click here to teach me more about this clue!

'stronghold' is the definition.
(I know that stronghold can be written as fastness)

'built with speed?' is the wordplay.
I cannot quite see how this works, but
'speed?' could be 'fast' (I've seen this before) and 'fast' is located in the answer.
The remaining letters 'ness' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'must be' acts as a link.

Can you help me to learn more?

(Other definitions for fastness that I've seen before include "speed?" , "Stronghold or fortress" , "Keep" , "Secure refuge" , "quality of not fading" .)

I've seen this clue in The Irish News.
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