その上

The phrase その上 is used to emphasize an additional piece of information. It can translate quite literally to the English equivalent "on top of that." It can also read as "in addition", "besides", or "moreover."

最近のケータイは写真が撮れるだけでなく、メールも出来る。その上、インターネットやテレビも見られる。これは、もう電話じゃない。
These days, not only can you take pictures with your cell phone, but you can also send email. On top of that, you can also do internet and watch TV. That's not a phone anymore.

リーさんはかっこうよくて、頭がよくて、その上、お金持ちだね。
Lee-san is handsome, smart, and moreover, he's rich. (I do not condone this~)

日本語の漢字は音読みと訓読みがあって、覚えるのが大変だ。その上、「一人」とか「今日」のような特別な読み方の漢字もあるので、漢字の勉強に時間がかかる。
Japanese kanji have on'yomi and kun'yomi readings, so it's tough to memorize. On top of that, because there are kanji that have specialized readings like 一人 and 今日, kanji studies take time.

誕生日に、彼氏はチョコレートとぬいぐるみをくれた。その上、好きなレストランにおごったんだ。
For my birthday, my boyfriend gave me chocolates and a stuffed animal. On top of that, he treated me to my favorite restaurant.