 The bell towers are the work of <unk> artist José <unk> Ortiz de Castro . They are capped with bell @-@ shaped roofs made of tezontle covered in <unk> , a white stone . Ortiz de Castro was in charge of the cathedral 's construction in the latter half of the 18th century until he died , unexpectedly . Manuel Tolsá of Valencia , who had built other notable buildings in Mexico City , was hired to finish the cathedral . At this point , the cathedral had already been 240 years in the making . He added the neo @-@ Classic structure housing the clock , the statues of the three theological virtues ( Faith , Hope , and Charity ) , the high <unk> surrounding the building , and the dome that rises over the transept . 
