。
One evening, I once stood atop this hill.
Literal
A-certain evening, I [topic-は] this hill [of-の] top [at-に] stood [experience-ことがある].
或る 'a certain' is a classical-literary variant of ある, reserved for formal or narrative writing and typical of phrasings like 或る朝 'one morning,' 或る人 'a certain person.' 夕暮 is a literary word for 'dusk/twilight,' sometimes written 夕暮れ with modern okurigana. ~たことがある is the 'have ever (done)' experience construction. The explicit 私は is marked, common in reflective literary narratives where the speaker foregrounds themselves against a described scene.