a long time ago; in those days
A set phrase used adverbially to refer to a distant past, often at the beginning of a story or reminiscence. Slightly literary or nostalgic in tone.
A long time ago, there was a big castle in this village.
In those days, there were no mobile phones.
Standard modern spelling.
Literary or old-fashioned kanji spelling using 其の for その.
Combination of その (that) and 昔 (old times, long ago). The phrase literally means 'that old time' and has been used since classical Japanese to refer to a distant past.