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in addition; furthermore
above (which); on top of (which)
「其の上」 has another reading with different meanings.
料金が高い。その上、サービスも悪い。
The price is high. In addition, the service is bad.
遠い。その上、交通も不便だ。
It's far. Furthermore, the transportation is inconvenient.
難しい。その上、時間もかかる。
It's difficult. Moreover, it takes time.