In ‘Alexander the Great’ by Philip Freeman - “Through the creative use of camels to terrify the Lydian cavalry, Cyrus took Sardis” “Alexander had no camels and no clear idea how to take Sardis” (pg 85)
How were these camels used as such an important military resource/strategy, even for Alexander two centuries later if he had them? How did they “terrify” enemies?
There isn't a whole lot behind the quote. I suspect Freeman meant it more as a casual remark on a curious aspect of ancient history than as a real assessment of siege tactics against the city of Sardis.
What he's referring to is the story, preserved in Herodotos (Histories 1.80), that Cyrus the Great used the camels in his baggage train to defeat the vaunted Lydian cavalry in battle, c. 547/6 BC. His men were no match for the Lydian mounted elite, so he ordered some of them to mount these pack animals and ride them into battle, knowing that horses were repelled by their sight and smell. The Lydian cavalry's horses duly fled from the camels, and so the battle was won and the Lydian capital Sardis captured.
The Greek author Xenophon repeats the same story in his much more detailed (though obviously fictionalised) account of the same battle. He seems to confirm that camels scared horses:
As soon as Artagerses saw Cyrus in action, he delivered his attack on the enemy's left, putting forward the camels, as Cyrus had directed. But while the camels were still a great way off, the horses gave way before them; some took fright and ran away, others began to rear, while others plunged into one another; for such is the usual effect that camels produce upon horses.
-- Xenophon, Education of Cyrus 7.1.27
We should probably trust the veteran commander Xenophon on the effect that camels had on horses, since we know that he encountered Persian camels in the flesh when he was fighting as a mercenary in Asia Minor. But the camels he saw were only used to carry baggage and were not used as an alternative form of cavalry. If they were ever used in battle, Herodotos is probably right that it would only be as an ad hoc measure.
In other words, there is no reason to connect the use of camels to the capture of Sardis in general. Cyrus' camels were a response to his opponent of the day, the elite Lydian cavalry. The fact that his victory allowed him to capture Sardis immediately after is only incidental. The Ionian rebels and their Athenian and Eretrian allies captured and burned Sardis in 498 BC despite their complete lack of camels. Alexander faced a very different situation after his invasion in 334 BC, and there's no reason to think a camel corps would have made it any easier for him to take Sardis.