The Citadel is an archive of information for George R.R. Martin's A Song of Ice and Fire.
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The ironborn come from a culture with a very strong warrior tradition -- much more so than mainland Westeros. The rest of the Seven Kingdoms have a warrior caste (the knights) on top of a larger base of peasants, farmers, craftsmen, merchants, etc. The "Old Way" of the islands encouraged almost all men (and some women, like Asha) to take up raiding, at least if they were young and healthy.
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