Complex sentence: Given its topography, the city's development in modern times was crucially dependent on the construction of numerous long high level bridges, whose construction in the 19th and 20th century at times stretched the limit of the possible.
Split into simpler sentences: Given its topography, the city's development in modern times was crucially dependent on the construction of long high level bridges. Their construction in the 19th and 20th century at times stretched the limits of the possible.