in addition; furthermore; moreover
Used to add information, often with a nuance of 'on top of that' or 'what's more'. Common in both spoken and written Japanese.
He is smart. Moreover, he is a hard worker.
This restaurant is cheap. On top of that, the food is good.
on top of that; above that
Literally 'on top of that', used to refer to something physically above or in addition to a previously mentioned thing. Often used in explanatory or descriptive contexts.
There is a book on the desk. On top of that, a pen is placed.
This restaurant is cheap. What's more, it's delicious.