Castiel sits next to 4 phonetic neighbors — Cassiel, Gabriel, and Daniel are the closest matches by sound. Most share Castiel's Hebrew-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"speed of God"
"gift, wealthy possessor"
"resolute protector's son"
"powerful ruler"
"garden, vineyard"
"colt keeper, young horse handler"
"wolf lover"
"long hill, ridge"
"peace, complete"
"thorny shrub, prickly plant"
"throne of the eye (possible etymology)"
"oarsman"
"chief desire"
"the fifth-born child"
"first zodiac sign, new beginnings"
"home on the wooded hill"
"messenger, herald"
"good man, strong man"
"true, faithful"
"well-born, of noble birth"
"trustworthy, loyal"
Similar sound: These names share phonetic patterns with Castiel — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Hebrew origins, giving them the same cultural heritage as Castiel.
Rhythm: Castiel is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.