Why is "the sanctuary city" translated as "la ciudad santuario"?

Is it the case that in Spanish, if using a noun (santuario) as an adjective, I somehow don't need to worry about gender agreement?

I was thinking that a more natural translation that avoids this is "la ciudad de santuario". But of course across the internet it seems the most common term is "ciudad santuario".

submitted by /u/naridimh
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