The dome , octagonal in shape , stand prominently on top of the centre of the cella . It was supported by four now @-@ destroyed columns which was located directly beneath it . The integrated bell tower have been built on top of the narthex , as was customary in contemporary Byzantine church architecture , and extend 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 .