Complex sentence: Like its 2nd-century model, the library is a round building fronted by a large two-storey portico which forms the main entrance on St Peter's Square, and is surrounded by five bays of Corinthian columns.
Split into simpler sentences: Like its 2nd-century model, the library is a round building fronted by a large portico. This two-storey portico forms the main entrance on St. Peter's Square, and is surrounded by five bays of Corinthian columns.