Complex sentence: Little is known of the appearance of the original church except that it was cross-shaped, but the historian Eusebius gives the following description of the mausoleum and grounds before Constantius' church was built: Eusebius Caesariensis.
Split into simpler sentences: Little is known of the appearance of the original church except that it was cross-shaped. Of the mausoleum and grounds, before Constantius' church was built, the historian Eusebius gives the following description: Eusebius Caesariensis.