Complex sentence: About the middle of the 1200s, these rooms were filled in and juniper posts were set into the ground to support beams and a roof, making a large, above-ground room of the courtyard.
Split into simpler sentences: About the middle of the 1200s, these rooms were filled in. Juniper posts were set into the ground to support beams and a roof, making a large, above-ground room of the courtyard.