It was originally intended that the the piazze basse be roofed so as to better protect troops, but the deafening noise and thick smoke caused by cannon fire led to the abandonment of this design in favour of an open type.
Original design
biographies

Piazza bassa on the Saint Francis side (now demolished) with casemates, 1900 - 1905 (G. Gerola, Vikelaia Municipal Library, Heraklion)
Panoramic view of the south piazza bassa in the Bethlehem Bastion, 2004 (photograph: Vassilis Kozonakis)