Used to shift the scene or introduce a parallel event, often in storytelling or narrative contexts.
Meanwhile, he was still working.
At the same time, a festival had begun in the town.
その頃 simply means 'at that time'; 一方その頃 combines it with a contrastive 'meanwhile' nuance.
Standard kanji-kana mixed spelling.
All-kana variant of the second part; less common but still used.
Compound of 一方 (ippō, 'one side; on the other hand') and その頃 (sono koro, 'at that time').