Complex sentence: Saint Catherine's, founded by Justinian in the early 6th c. on the site of a prior establishment by Helena of Constantinople in 313 or thereafter, lies at the foot of Gebel Musa where legend has it that Moses saw the Burning Bush.
Split into simpler sentences: Saint Catherine's was founded by Justinian in the early 6th CE on the site of a prior establishment named for Helena of Constantinople founded about 313CE. The site lies at the foot of Gebel Musa where legend has it Moses saw the Burning Bush.