The dome itself is a rather irregular mixed masonry construction of tuff blocks, bricks and mortar, covered with lead sheets. There is a thin inner shell made of bricks which can be a later addition to create a suitable surface for the frescoes. The dome is capped with an elegant globe and cross finial. The high brick tambour is punctuated with eight arched windows which originally had stone mullions like all the other openings of the church. The whole structure rests on a low square base.