Complex sentence: This secondary entrance to the house, intended only for use by servants and tradesmen is reached by a descending, segmented external double staircase, a common enough feature in a baroque house ascending the principal entrance, but rare when descending to a secondary entrance.
Split into simpler sentences: This secondary entrance to the house, intended only for servants and tradesfolk is reached by a descending, segmented external double staircase. This is a common feature in a baroque house ascending the principal entrance, but rare when descending to a secondary entrance.