This is the 18170th most frequent Korean word.
수호했던
"수호했던" means "protected" or "defended" (past tense).
In this sentence, '수호했던' describes the act of protecting something, here referring to the act done by the described hero.
그는 마을을 수호했던 영웅으로 알려져 있습니다.
He is known as the hero who protected the village.
Here, '수호했던' modifies '기억들' to indicate that these are memories of protective actions or events.
수호했던 기억들이 시간이 지나면서 희미해졌습니다.
The memories of protection faded over time.
In this context, '수호했던' is used to describe the role associated with protective actions or duties.