Original:  In 215 Caracalla travelled to Alexandria and responded to this insult by slaughtering the deputation of leading citizens who had unsuspectingly assembled before the city to greet his arrival  , before setting his troops against Alexandria for several days of looting and plunder.
Revised:  Caracalla responded to this insult by slaughtering the deputation of leading citizens who had unsuspectingly assembled before the city to greet his arrival , before setting his troops against Alexandria for several days of looting and plunder.
Edit intent: coherence