Complex sentence: The overhead monitors were stacked four rows high, twice as high as the existing trading pits, in order to maximize the use of the narrow area, although these were removed later as a result of seismic concerns and replaced with counter top flat screens.
Split into simpler sentences: The overhead monitors were stacked four rows high, twice as high as the existing trading pits, in order to maximize the use of the narrow area. Eventually the hundreds of overhead monitors were removed because of seismic concerns and replaced with counter top flat screens.