Complex sentence: The main staircase is made of 21 relatively low, wide steps of single blocks on which the 21 steps are relatively low and wide, and the wrought-iron banister was made by the same workshop that supplied the entrance gates.
Split into simpler sentences: The main staircase is made of 21 relatively low, wide steps of single blocks. The wrought-iron banister was made by the same workshop that supplied the entrance gates.