Complex sentence: Riding master Yves Bienaimé had begun his career in the riding school housed in stables in 1959, and had been struck by the state of neglect of the still impressive building.
Split into simpler sentences: Riding master Yves Bienaimé had begun his career in 1959, in the riding school housed in the stables. Twenty years later, he was struck by the state of neglect of the still-impressive building.