 The dome , octagonal in shape , stands prominently on top of the centre of the cella . It was supported by four now @-@ destroyed columns which were located directly beneath it . The integrated bell tower has been built on top of the narthex , as was customary in contemporary Byzantine church architecture , and extends from the rectangular main structure . The bell tower was originally rectangular , though it is now partially ruined . It was reached from the south by means of a stone staircase . 
