In the 1930s, Koestler worked mostly as a journalist, regularly reporting from abroad (wherever "abroad" is for someone who moved from Hungary to Austria to Germany to France) and writing other things (more specifically, sex encyclopaedia) just to survive financially. Based on this, I wonder how much time he found to research the history of the Third Servile War. More specifically, how historically accurate is The Gladiators? There are at least two near-contemporary sources about the war, one by Appian and one by Plutarch. Are the events in Koestler's novel closer to one of these sources than to the other (or the other ones)?
An answer to this question might go into quite a lot of detail, but for the purpose of this site, the level of detail found in Wikipedia's plot summary of the novel would be sufficient. (Obviously, the answer should rely on more reliable sources than Wikipedia.)