Ancient Worlds - 8 free documentaries about gladiators & the colosseum.
Start with The Colosseum’s Rise and Fall (55 min, PBS), then Spartacus: Rise of the Gladiator (44 min, National Geographic). All 8 documentaries here are free to watch from their makers' official uploads. To read deeper, the top-rated book on gladiators & the colosseum is Gladiators by Fik Meijer.