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Ideas abounded from every quarter, however Salman proposed a plan that had been
both used and proven effective in Persia. Salman advised the Prophet (salla Allahu alihi
wa sallam) that when the Persians feared a cavalry attack, they would dig a large,
circular trench around them as it was extremely difficult for the horses of the enemy to
cross the wide divide and consequently they were better able to defend themselves.
Salman's proposal met with great enthusiasm and so it was agreed that this would be
the best course to adopt.

The Strategic Importance of the Jewish Fortresses of the Krayzah

It was unnecessary to dig a complete trench around the entire City as there was an
unbroken line of fortified houses, strong enough to resist the advance of the enemy in
one part. Then again, outside the City lay the fortresses of their allies from the Jewish
tribe of Krayzah which also afforded them additional protection.

There was yet another blessing, outside the City towards the north-west lay
impenetrable hillocks of rock. One of these hillocks was called the hill of Sila, and it was
there that they decided to make their camp after connecting the existing fortifications