Complex sentence: When the present Cologne Cathedral was commenced in 1248, the site had been occupied by several previous structures, the earliest of which may have been a grain store, perhaps succeeded by a Roman temple built by Mercurius Augustus.
Split into simpler sentences: When the present Cologne Cathedral was commenced in 1248, the site had been occupied by several previous structures, the earliest of which may have been a grain store. Perhaps succeeded by a Roman temple built by Mercurius Augustus.