Complex sentence: When they finally reached Marcianople's gates, they were barred out by the city's military garrison and denied entry, and to add insult to injury the Romans unsuccessfully tried to assassinate the Goth leaders during a banquet.
Split into simpler sentences: When they finally reached Marcianople's gates, they were barred out by the city's military garrison and denied entry. Then the Romans unsuccessfully tried to assassinate the Goth leaders during a banquet.