You need to finish the gold mine first, as well as Gin's route if you have male partners enabled.
Thing is I've already done the gold mine quest and I don't have male partners enabled, but I still can't access the arena...
go to the entrance of the city, and there will be an announcement!
I've already tried exploring the whole city and there's no announcement. I've also tried to talk to several NPCs, but again nothing happens.Perhaps my save is bugged?
No need to talk to anyone. Have you met Gin before? If so, you'll need to finish his route before anything triggers.
