Complex sentence: The door to the right opened on to the upper floor and the staircase; the door to the left was that of the guest room he held prepared for Gauguin; the window in the front wall looked on to Place Lamartine and its public gardens.
Split into simpler sentences: The door to the right was opening to the upper floor and the staircase; the door to the left served the guest room he held prepared for Gauguin. The window in the front wall was looking to Place Lamartine and its public gardens.