insuper

Insuper is in plus super, and it keeps both halves of the sense. Physically it means on top or above, of a thing laid over another. In argument it means moreover, besides, into the bargain, and it usually carries a faint edge: what follows insuper is often one thing too many.

It is an adverb and governs no case in classical Latin, though late and medieval writers let it slip into a preposition with the accusative, and you will meet that here. Where an English writer says "and what is more", a Roman has insuper, praeterea or ad haec to choose from. Do not read it as the compound of insum, which is a different verb entirely.