Complex sentence: St. Margaret's Chapel within Edinburgh Castle has been traditionally regarded as Edinburgh's oldest building, though most scholars now believe that in its surviving form it was more likely built by Margaret's youngest son David I in his mother's memory.
Split into simpler sentences: St. Margaret's Chapel within Edinburgh Castle has been traditionally regarded as Edinburgh's oldest building. Most scholars now believe that the chapel, as seen today, was more likely built by Margaret's youngest son David I in his mother's memory.