at that vs on top of that

at that

prep
  • In addition to what has been said; furthermore; moreover. 

  • Now that it has been mentioned. 

  • Thereupon; directly after, and as a result of, that. 

on top of that

prep
  • Also, additionally, moreover. 

How often have the words at that and on top of that occurred in a corpus of books? (source: Google Ngram Viewer )